Examining relationships between sleep posture, waking spinal symptoms and quality of sleep: A cross sectional study.

INTRODUCTION: Research with a focus on sleep posture has been conducted in association with sleep pathologies such as insomnia and positional obstructive sleep apnoea. Research examining the potential role sleep posture may have on waking spinal symptoms and quality of sleep is however limited. The aims of this research were to compare sleep posture and sleep quality in participants with and without waking spinal symptoms.
METHODS: Fifty-three participants (36 female) were, based on symptoms, allocated to one of three groups; Control (n = 20, 16 female), Cervical (n = 13, 10 female) and Lumbar (n = 20, 10 female). Participants completed an online survey to collect general information and patient reported outcomes and were videoed over two consecutive nights to determine sleep posture using a validated classification system including intermediate sleep postures.
RESULTS: Participants in the symptomatic groups also reported a lower sleep quality than the Control group. Compared to Control group participants, those in the Cervical group had more frequent posture changes (mean (SD); 18.3(6.5) versus 23.6(6.6)), spent more time in undesirable/provocative sleep postures (median IQR; 83.8(16.4,105.2) versus 185.1(118.0,251.8)) minutes and had more long periods of immobility in a provocative posture, (median IQR: 0.5(0.0,1.5) versus 2.0 (1.5,4.0)). There were no significant differences between the Control and Lumbar groups in the number of posture changes (18.3(6.5) versus 22.9(9.1)) or the time spent in provocative sleep postures (0.5(0.0,1.5) versus 1.5(1.5,3.4)) minutes. DISCUSSION: This is the first study using a validated objective measure of sleep posture to compare symptomatic and Control group participants sleeping in their home environment. In general, participants with waking spinal symptoms spent more time in provocative sleep postures, and experienced poorer sleep quality.

Introduction

Sleep is considered essential for human mental and physical recovery [1-3], with some people going to bed in pain, to wake recovered. Nonetheless, a proportion of the population who are asymptomatic when retiring wake with spinal symptoms and others with existing spinal symptoms wake with exacerbations of their retiring spinal symptoms [4-7].

Spinal symptoms are common and mostly occur in the cervical and lumbar regions, with a one-year point prevalence of 30 to 50% for cervical pain [8] and 38% for lumbar pain [9]. The prevalence of both cervical and lumbar pain has increased markedly (cervical 21.1% and lumbar 17.3%) over the past 25 years, and these rates are expected to continue rising [10]. Other types of symptoms like stiffness and bothersomeness, still important to patients, are less well documented.

It has been postulated that poor sleep posture during the night may be responsible for the production of waking cervical [11-13] and lumbar symptoms [14]. It was determined in young military recruits, that 33% had their most intense spinal pain during sleep hours or on waking and that for 50% of the recruits, the spinal pain was significant enough to cause disruption to their sleep routine [4].

Anecdotal and theoretical evidence suggests that mechanical loads induced by some sleep postures, like prone, may provoke spinal pain [6, 15]. Collagen containing tissues like ligaments, intervertebral discs and capsules, undergo predictable mechanical and viscoelastic changes such as creep, hysteresis and fatigue failure in response to a single sustained load or to repeated loads [16, 17]. Loads sustained for greater than 10 minutes and repeated loads causing 3% or greater elongation, have resulted in collagenous tissue micro-damage. Micro-damage has been associated with an increased expression of pro-inflammatory cytokines in animal studies [18]. Muscle spasms associated with sustained flexion or rotation spinal postures have been reported in both human [19, 20] and animal studies [18]. It therefore seems plausible, that sleep postures sustained for greater than 10 minutes or repeated, may cause micro damage and result in spinal symptoms.

Sleep posture has also been associated with sleep quality. Poor quality of sleep is subjectively determined by delayed sleep onset, more awakenings after sleep onset, increased total wake time, and poor continuity of sleep [21, 22]. Therefore, factors like sleep posture that provokes spinal pain, potentially causing increased total wake time, could impact on sleep quality. Poor sleep quality is significantly associated with adverse health outcomes for adults [23, 24] and is predictive of musculoskeletal pain in pre-adolescents, adolescents, young adults [25-28] and adults without pain [24]. For this reason, it is important to identify any factor that potentially could adversely affect an individual’s ability to maintain an asleep state.

The aims of this research were to determine whether there were differences in sleep posture and sleep quality in participants with and without waking spinal symptoms.

Methods

This study was approved by the Curtin University Human Research Ethics Committee (HR 140/2014). Approval to share data beyond the investigators was not obtained. Written informed consent was obtained from all participants. The study was registered with the Australian New Zealand Clinical Trials Registry on 4/07/2014 (ACTRN12614000708651). The subject in Fig 1 has given written informed consent.

Fig 1

Sleep posture classification based on plausible spinal load.

Sustained postures like rotation, have been identified as causing tissue microdamage and muscle spasms.

Recruitment occurred in Esperance, a rural town of Western Australia through word of mouth, recruitment posters, radio interviews, letters to possible referrers and newspaper advertisements. Volunteers were asked screening questions in a phone or face to face interview to determine eligibility for the study. Volunteers who were younger than 18 years or older than 46 years were excluded. The younger age was for legal consent reasons and the upper age to minimise the chances of confounding factors like increasing severity of spinal degenerative changes [29]. Volunteers with medical conditions such as severe osteoarthritis, spinal stenosis, oesophageal reflux and late stage pregnancy or using devices such as breathing apparatus that prevented them from sleeping in all postures were excluded [30]. Those with co-existing medically diagnosed inflammatory conditions or unremitting pain (e.g., rheumatoid arthritis, neuropathic pain) were also excluded. Volunteers using medically prescribed hypnotic or relaxant medications would have been excluded as these medications can alter frequency of posture changes during sleep, but none were excluded for this reason. A total of 53 participants (36 female) with predominately morning symptoms of pain, stiffness or bothersomeness were recruited over a period of 2.5 years. Using a process of best fit, eligible volunteers were allocated to one of the three groups, Control (n = 20, 16 female), Cervical (n = 13, 10 female) and Lumbar (n = 20, 10 female). Participants with spinal pain, stiffness or bothersomeness greater than or equal to 3 out of 10 on a numerical rating scale (NRS) [31], that occurred four or more times per month and decreased within 60 minutes of waking, were allocated into a symptomatic group (Cervical or Lumbar) dependent on their self-reported dominant area of symptoms. Participants without symptoms, or with symptoms less frequently than four times per month, or less than 3 out of 10, were allocated to the Control group. Each symptomatic group was individually compared with the Control group.

Outcome measures

Due to the vanguard nature of this study, a broad range of pain, disability, sleep and quality of life patient reported outcome measures were collected to facilitate better understanding of possible relationships between sleep posture and waking spinal symptoms. Participants in all three groups were emailed a link to a Survey Monkey questionnaire, enabling the online completion of baseline information (e.g., age, gender, body mass index, medications, level of education, and self-reported sleep posture) and patient reported outcome measures. Data for the following patient reported outcome measures was collected.

Numerical rating scale

Numerical rating scales for waking pain, stiffness, bothersomeness and quality of sleep in the prior 2 weeks. Higher scores indicated increased symptoms for pain, stiffness and bothersomeness, whereas a higher score indicated a better quality of sleep.

Neck Disability Index (NDI)

A 10-item, self-reported questionnaire measuring cervical disability. In a group of patients with non-specific neck pain, a minimal clinically important difference of 3.5 points best distinguished those patients who were clinically improved from those who were not [32].

Spine Functional Index (SFI)

A 10-item, 2 weeks recall whole of spine functional measure, the SFI-10 demonstrated high criterion validity with the Functional Rating Index (r = .87), equivalent internal consistency (α = .91) and a single-factor structure in patients with spinal pain referred to physiotherapy clinics by medical practitioners [33, 34].

Roland Morris Disability Questionnaire (RMQ)

A 24-item, immediate recall self-reported lumbar disability measure, found to be reliable, valid and responsive to change over time. Higher scores indicated greater disability.

Hospital Anxiety and Depression Scale (HADS)

A 14-item, two domain assessment widely used to identify cases of anxiety and depression in non-psychiatric hospital clinics for adults greater than 16 years of age [35]. Each item is scored on a 4-point Likert scale (0–3) generating anxiety and depression scores ranging from 0 to 21.

Short Form—36 (SF-36)

The Short Form-36 is a well-validated measure of health status. Version 2 (1 week recall) was used with Australian normative data [36, 37] to calculate two summary scores (i.e., Physical Component and Mental Component Score). A higher score indicates a better health status.

Insomnia Sleep Index (ISI)

A 7-item, 2 weeks recall self-reported scale designed to assess the nature, severity and impact of insomnia and to monitor treatment response in adults [38, 39]. Scale development included a heterogenous group of patients with insomnia secondary to pain conditions [40].

Pittsburgh Sleep Quality Index (PSQI)

A 19-item, 7-domain, 2 weeks recall self-reported questionnaire which examines subjective sleep quality, with global scores ranging from 0 to 21. A global PSQI score greater than 5 is considered a poor sleeper and yielded a diagnostic sensitivity of 89.6% and specificity of 86.5% (kappa = .75, p < .001) in distinguishing between good and poor sleepers. Higher scores indicate poorer sleep quality [41, 42].

Sleep posture

The classification of sleep posture into supine, supportive side lying, provocative side lying and prone (Fig 1), and the method used to collect sleep posture data has been previously described [43]. The validity and reliability of this method has also been previously reported [44]. In brief, two IR cameras were installed in the participant’s bedroom on portable stands. One camera was placed at the foot end of the bed, and the other centrally over the bed. These IR cameras were cable-connected to a digital data recorder. A monitor was connected to the data recorder to optimise camera viewing fields. Data collection automatically commenced at 2000hrs and stopped at 0800hrs. Participants were encouraged to maintain their normal sleeping routines.

After two consecutive nights video equipment was retrieved and recordings were reviewed. Head, trunk and leg positions were noted and the overall sleep posture was recorded as supine, supportive side lying, provocative side lying or prone. Each posture change was manually recorded relative to the time stamp and rounded up to the next half minute. For example, 1 to 29 seconds became a half minute and 31 to 59 seconds became a full minute. To be recorded, a posture needed to be sustained for at least 1 minute. Head movements from neutral to right or left rotation, without a change in trunk position were recorded, but were not considered a new posture because of no major change in load on the spine. If participants moved from right to left supported side lying or provocative side lying, this was recorded as a new posture, due to the major change in body posture. Sustained posture intervals of 30 minutes or greater were described as long periods of postural immobility Researchers have indicated that long periods of postural immobility are an indication of sleep stability [45, 46]. Exploring the idea that some sleep postures may be provocative of spinal symptoms, our study sought to not only measure the frequency of long periods of postural immobility (these will be referred to as standard long periods of postural immobility), but also the posture in which the long periods of postural immobility occurred and the number of 30-minute periods for each long periods of postural immobility (referred to as actual long periods of postural immobility). That is, a posture held for 65 minutes would be recorded as one standard long period of postural immobility but two (30-minute) actual long periods of postural immobility.

In addition to the collection of video data, participants completed a Morning After Questionnaire each morning after being videoed, to score pain, stiffness, bothersomeness and quality of sleep on a NRS in relation to the prior night.

Data

Sample size

A priori sample size calculations were based on data collected in a pilot study [43]. It was calculated a sample of 30 in each group (i.e., Control, Cervical, Lumbar) would have a power of 99%, assuming a two tailed p-value of .05 to detect a large effect (0.8). Further, a sample of 60 people with symptoms would have sufficient size to detect a minimal clinically important change in pain of 1.5 points on a NRS assuming a standard deviation (SD) of 2 points, with a power of 99% and a minimal clinically important change of 5 points on the SF36 summary scales, assuming a SD of 10 points with a power of 96% following an intervention. The sample sizes of 30 for each group were not achieved, rather final sample sizes for each group were Control (20), Cervical (13) and Lumbar (20).

Data analysis

Questionnaires were scored according to published scoring algorithms or instructions provided by developers. Sleep data from Night 1 and Night 2 were averaged prior to further analyses. Statistical analysis was performed using IBM® SPSS® v24.0. All data were checked for outliers by visual inspection of boxplots or population pyramids. Outlying data points were checked for data entry errors and measurement errors.

On initial review of raw video data and before data analysis commenced, outliers within groups were identified. It became apparent, that while some participants were by definition sleeping in provocative side lying (i.e., top thigh advanced forward of the bottom thigh, Fig 1), because of the use of pillows and/or duvet to support the upper thigh, participants technically did not induce spinal rotation or extension. For this reason, the provocative side lying sleep postures of three participants were reclassified to supportive side lying. Genuinely unusual values were rare and retained in the analyses.

Descriptive statistics were presented as count and percentage, mean (M) and standard deviation (SD) or median and interquartile range (IQR). A range of sleep posture variables were extracted from the video data to examine possible relationships with waking spinal symptoms; specifically, the percentage of time and the total amount of time spent in each sleep posture (i.e., supine, supportive side lying, provocative side lying and prone). For some analyses, postures were grouped into supportive (i.e., supine and supportive side lying) and provocative (i.e., provocative side lying and prone), based upon plausible spinal load. The distribution of the data was examined using numerical (Shapiro-Wilk test) and graphical (visual examination of Histograms and Normal Q-Q Plots) methods. Achieving a normal distribution was not possible for most Control group variables, particularly patient reported outcome variables, as most participants in this group reported low or no symptom levels. After outliers and normality assumptions were checked, homogeneity of variance was checked using the Levene statistic for normally distributed data (p > .05 significant). Between group comparisons (Cervical versus Control and Lumbar versus Control) were undertaken using a one-way analysis of variance (ANOVA) statistic (F), or for non-normally distributed data, a Mann-Whitney U test (U). A chi-square test was used to compare categorical variables between groups. A p < .05 (two tailed where appropriate) was considered significant for all analyses.

Results

Group characteristics

The age of participants ranged from 18 to 45 years, with the largest group of participants in the 41 to 45 years range. Overall there were more female than male participants, with 16 females in the Control group and 10 in both the Cervical and Lumbar groups. There were no significant differences between groups in distribution of age, gender, education or BMI scores (Table 1). Participants nominated the types of medications and supplements they were currently using. Approximately the same percentage in each group used none, one to two and three or more medications or supplements. The types of medications and supplements used in each group are detailed in Table 1.

Table 1

Sample characteristics of age, gender and medication use.

	Control (n = 20)	Cervical (n = 13)	Lumbar (n = 20)	Total (n = 53)
Age
18–20	2 (4%)	0 (0%)	1 (%)	3 (6%)
21–25	3 (6%)	2 (4%)	4 (8%)	9 (17%)
26–30	2 (4%)	1 (2%)	2 (4%)	5 (9%)
31–35	3 (6%)	1 (2%)	1 (2%)	5 (9%)
36–40	1 (2%)	6 (11%)	3 (6%)	10 (19%)
41–45	9 (18%)	3 (6%)	9 (17%)	21 (40%)
Gender
Male	4 (8%)	3 (6%)	10 (19%)	17 (32%)
Female	16 (30%)	10 (19%)	10 (19%)	36 (68%)
Type of Medication
Pain relief	0 (0%)	6 (15%)	2 (5%)	8 (20%)
Antidepressant/Anxiety	2 (5%)	3 (8%)	2 (5%)	7 (18%)
NSAID	1 (2%)	1 (2%)	1 (2%)	3 (8%)
Other*	12 (30%)	4 (10%)	6 (15%)	22 (55%)

Note. Other* includes medications for birth control, anti-reflux, high blood pressure, high cholesterol, underactive thyroid, hay fever and mineral supplements>

Patient reported outcomes

Participants in the Cervical group had significantly worse pain, stiffness, and bothersomeness on waking than the Control group (Table 2). They also recorded significantly poorer scores in all of the patient reported outcome questionnaires except the SF-36 MS (Table 2).

Table 2

Comparisons between Control and symptomatic groups for waking symptoms in the prior 2 weeks and patient reported outcomes.

Variable	Control (n = 20)	Cervical (n = 13)	Lumbar (n = 20)
Waking Symptoms in Prior 2 Weeks
Pain b	0.0 (0.0, 1.0)	5.0 (3.5, 6.0) **, U	4.0 (3.0, 5.0) **, U
Stiffness b	1.0 (0.0, 2.0)	5.0 (3.0, 6.0) **, U	5.0 (3.2,6.8) **, U
Bothersomeness b	1.0 (0.0, 2.0)	6.0 (3.5, 7.0) **, U	4.0 (3.0, 6.0) **, U
Patient Reported Outcomes
RMQ b	0.0 (0.0, 0.0)	N/A	2.0 (1.0, 5.0) **, U
Neck Disability Index b	2.0 (0.0, 6.0)	18 (12.0, 26.0) **, U	N/A
Spine Functional Index b	98.0 (93.0, 100)	86.0 (68.0, 94.0) **, U	80.0 (62.5, 91.5) **, U
SF-36 PS b	69.3 (62.9, 71.8)	58.6 (52.4, 67.8) *, U	59.9 (50.7, 68.8) *, U
SF-36 MS b	83.5 (78.6, 86.4)	80.0 (63.8, 84.45) U	77.2 (68.9, 82.8) *, U
HADS–Anxiety a	2.5 (2.3)	5.3 (2.8) *, F	6.4 (4.8) *, F
HADS–Depression b	1.0 (0.0, 2.8)	3.0 (2.0, 4.0) *, U	2.0 (0.2, 6.0) U

Note. RMQ = Roland-Morris Disability Questionnaire, HADS = Hospital Anxiety and Depression Scale, SF-36 PS = Short Form 36 Physical Score, SF-36 MS = Short Form 36 Mental Score

** p < .001 compared with Control group

* p < .05 compared with Control group

F = ANOVA

U = Mann-Whitney.

Data are

a Mean (Standard Deviation) or

b Median (Interquartile Range).

Participants in the Lumbar group had significantly worse pain, stiffness, and bothersomeness on waking than the Control group (Table 2). They also recorded significantly poorer scores in all of the patient reported outcome questionnaires except the HADS—Depression (Table 2).

Comparison of sleep posture variables

Compared with the Control group, participants in the Cervical group spent a greater percentage of the night in provocative side lying and combined provocative sleep postures. When time in each posture was expressed in absolute values (minutes), the Cervical group spent, on average, twice as long in provocative side lying (Table 3). Participants in the Cervical group changed their sleep postures more frequently than the Control group and spent more of their long periods of postural immobility in provocative sleep postures (Table 3).

Table 3

Comparisons between the Control and Symptomatic groups in time spent in each posture, expressed as a percentage of the total time in bed (percentage time) or in minutes (total time) and posture mobility.

Variable	Control (n = 20)	Cervical (n = 13)	Lumbar (n = 20)
Percentage Time
Supine a	39.2 (17.3)	32.8 (17.3) F	34.5 (16.8) F
Supportive side lying a	43.0 (17.7)	31.2 (15.1) F	38.8 (16.1) F
Provocative side lying b	16.0 (2.2, 22.4)	29.6 (12.7, 48.8) *, U	23.0 (6.6, 33.6) U
Prone b	1.9 (0.0, 4.0)	6.4 (0.0, 12.1) U	3.9 (0.0, 5.3) U
Supportive Combined b	82.2 (74.6, 96.0)	64.0 (49.3, 75.5) *, U	73.1 (63.6, 92.0) U
Provocative Combined b	17.8 (4.0, 25.4)	36.0 (24.5, 50.6) *, U	26.9 (8.0, 36.3) U
Total Time (minutes)
Supine a	178.6 (75.9)	170.8 (85.1) F	164.0 (70.9) F
Supportive side lying a	200.8 (91.6)	158.1 (83.1) F	193.2 (88.8) F
Provocative side lying b	75.0 (10.2, 97.5)	153.0 (63.7, 242.5) *, U	115.3 (34.6, 186.8) U
Prone b	8.8 (0.0, 17.0)	32.1 (0.0, 59.5) U	19.2 (0.0, 24.8) U
Supportive Combined b	379.5 (326.2, 453.9)	328.9 (254.2, 396.2) U	357.2 (301.1, 447.6) U
Provocative Combined b	83.8 (16.4, 105.2)	185.1(118.0, 251.8) *, U	134.6 (40.8, 199.0) U
Posture Immobility (number)
Posture changes a	18.3 (6.5)	23.6 (6.6) *, F	22.9 (9.1) F
Standard LPPI b	5.0 (3.6, 6.4)	6.0 (5.5, 6.5) U	6.0 (1.5) F
Actual LPPI a	7.8 (2.4)	8.3 (1.3) F	8.5 (6.0, 9.9) U
Supportive LPPI a	6.6 (2.7)	5.5 (1.9) F	6.0 (4.1, 8.1) U
Provocative LPPI b	0.5 (0.0, 1.5)	2.0 (1.5, 4.0) *, U	1.5 (1.5, 3.4) U
Sleep
Quality of sleep a (NRS)	6.5 (2.1)	4.8 (1.7) *, F	4.6 (2.2) *, F
Insomnia Sleep Index b	3.0 (1.0, 4.0)	6.0 (4.5, 9.5) *, U	10.0 (5.2, 11.8) **, U
PSQI a	3.2 (1.6)	7.4 (4.1) *, F	6.7 (3.1) **, F

Notes

F = ANOVA

U = Mann-Whitney

* p < .05 compared with Control; LPPI = Long periods of postural immobility; Standard LPPI = the number of times one posture is maintained for ≥.30 minutes; Actual LPPI = the number of 30 minute periods postures are maintained; PSQI = Pittsburgh Sleep Quality Index.

Data are

a Mean (Standard Deviation) or

b Median (Interquartile Range).

There was no statistically significant difference (p = .052) between the time participants in the Lumbar group spent in provocative sleep postures compared with the Control group (Table 3). Nor were there any statistically significant differences between the Lumbar and Control groups in postural immobility (p > .07) (Table 3).

Comparison of sleep quality variables

With respect to sleep quality, both symptom groups self-reported significantly lower sleep quality than the Control group using the numerical rating scale (Table 3). Scores from the Insomnia Sleep Index and Pittsburgh Sleep Quality Index also indicated poorer sleep quality in both the Cervical and Lumbar groups (Table 3). For the Cervical group however, the average Insomnia Sleep Index score remained within the ‘no clinically significant insomnia’ band of scores whereas the Lumbar group, on average, would be classified as having ‘subthreshold insomnia’. Both groups would be classified as ‘poor sleepers’ using the Pittsburgh Sleep Quality Index classification.

Discussion

Volunteers for this study were allocated into one of two symptomatic groups (Cervical or Lumbar) or a Control group based on the location, duration and intensity of their self-reported symptoms. As would be expected with classification of this nature, the symptomatic groups had greater morning symptom scores and decrements in the various patient reported outcomes measured (summarised in Tables 1 and 2).

The findings of the study are consistent with the theories posed prior to the study; that people with spinal pain would spend more of the night in provocative sleep postures and would have lower sleep quality than a Control group. Interpretation of these findings must be in the context of the study design and limited sample size. Given the cross-sectional study design, it is not possible to be sure whether any of the variables of interest are causal or whether their presence is simply coincidental. Small studies are more vulnerable to Type II errors than larger studies.

The participants with morning symptoms of neck pain or stiffness slept differently to those without morning symptoms. In comparison with the Control group, participants in the Cervical group spent significantly more time in provocative sleep postures. These results are similar to an epidemiological study examining waking cervical symptoms and sleep posture [6]. In that study, participants who reported prone as their dominant sleep posture also reported the highest percentage of waking cervical symptoms. An interesting consideration is whether it is the amount of time or the percentage of time spent in provocative sleep postures that is more likely to provoke symptoms. With regards to the Cervical group, we found both the percentage time and the total time spent in provocative postures was greater than the Control group. A study conducted on feline spines points towards the amount of time as being important and once this threshold is passed, recovery of tissue takes proportionally much longer [47].

We examined the frequency of posture shifts from the stand point of plausible tissue load. One possible reason to change posture more frequently would be to offload pain sensitive structures more likely to be aggravated by sleep postures such as prone and provocative side lying. Spinal and capsular ligaments are highly innervated [48] and have been shown to produce pro-inflammatory cytokines following sustained or repeated loading in feline studies [18]. Pain free adults of mixed age and gender have been noted to change posture approximately 12 to 20 times per night [49-51]. This frequency of posture shifts is reported to double in those describing themselves as poor sleepers [22]. In comparison to our Control group, the Cervical group did experience a significantly higher frequency of posture shifts.

We did not find any differences in Actual or Standard long periods of postural immobility between the Control and Symptomatic groups using the common long period of postural immobility definition of a 30-minute interval. Given other researchers have noted that spinal tissue creep occurs within 10 minutes [19, 20], to be more sensitive with respect to plausible spinal tissue loading, perhaps the concept of a long period of postural immobility needs to be modified to a shorter time-period. To our knowledge, we are the first group to examine the relevance of long periods of postural immobility in relation to provocative and supportive sleep postures. In comparison to participants in the Control group, participants in the Cervical group spent more long periods of postural immobility in provocative sleep postures. This result runs contrary to the theory that long periods of postural immobility are postures of comfort and therefore sustained for longer periods of time. It is however consistent with our theory, that sustained sleeping in provocative postures could cause waking spinal symptoms. Rather than reflecting postures of comfort, it may be that postures adopted for long periods are postures of habit. These postures may be amenable to educational interventions like posture retraining [52, 53].

Participants in the Lumbar group spent three times as long as those in the Control group in provocative postures, but results were not statistically significant. Visual examination of the data in Table 3 reveals that values for the Lumbar group consistently fell between those for the Control and Cervical groups, never reaching statistical significance. It is possible that we did not have sufficient statistical power to identify a difference between the Control and Lumbar groups causing a Type II error. It is also possible that, as a result of our recruitment criteria, the differences between our Control and Lumbar groups were reduced. For example, at enrolment, 45% of participants in the Control group nominated having some low back pain in the prior 2 weeks, however their level of reported pain was insufficient to meet the criteria for allocation into the Lumbar group.

In this study, sleep quality was measured in three ways; numerical rating scale for sleep quality over the prior 2 weeks, the Pittsburgh Sleep Quality Index and the Insomnia Sleep Index. The latter measures are commonly used in the sleep literature and quantify different aspects of sleep quality. Good quality sleep has been associated with a side lying sleep posture [6]. However, given the wide variety of side lying sleep postures, it is likely that some side lying postures may place adverse loads on spinal tissues and not be conducive to good quality sleep.

The relationship between sleep quality and pain has historically been considered bidirectional. More recent research points to sleep quality as being the antecedent factor [54] and in the insomnia research literature, poor sleep quality is considered predictive of new pain onsets and exacerbation of existing pain [55, 56]. If sleep posture influences sleep quality, then optimising sleep posture could potentially reduce spinal pain via two separate mechanisms. Firstly, by reducing collagenous spinal tissue load and injury associated with creep, and secondly, by improving sleep quality.

Neck and low back pain are global health problems and the challenge to identify risk factors has been highlighted [57, 58]. Determining modifiable risk factors could assist in the identification of individuals predisposed to spinal symptoms and assist in the development of appropriate education and prevention strategies. In a recent systematic review examining risk factors for first episode neck pain, the most significant physical risk factor was an awkward, sustained posture [57]. When examining trigger events (i.e., brief exposures) that precipitate acute low back pain, symptom onset was most common in the morning [59]. The timing implicates sleep posture as a possible factor and passive collagenous restraints, (e.g., IVD, ligament, joint capsule) as being the tissues most likely affected [60]. Sleep posture was not explored in either study. Further, the lack of research focus on spinal symptoms associated with sleep posture was highlighted in a recent scoping review, in which only four studies were found to address these topics [61].

Conclusion

Currently, it is not known if sleep posture is a risk factor for acute onset or recurrent spinal symptoms, but this study has demonstrated that participants with symptoms of cervical pain and stiffness in the morning spent more of the night in provocative sleep postures. It is plausible that a similar association exists for people with symptoms in the low back, however this was not confirmed in this study. Both symptomatic groups had poorer sleep quality. Future exploration of the effects of provocative sleep posture and potential benefits of sleep posture education and modification seem justified.

8 Feb 2021

PONE-D-20-35066

Examining relationships between sleep posture, waking spinal symptoms and quality of sleep: A cross sectional study

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We will update your Data Availability statement to reflect the information you provide in your cover letter.

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As per the PLOS ONE policy (http://journals.plos.org/plosone/s/submission-guidelines#loc-human-subjects-research) on papers that include identifying, or potentially identifying, information, the individual(s) or parent(s)/guardian(s) must be informed of the terms of the PLOS open-access (CC-BY) license and provide specific permission for publication of these details under the terms of this license. Please download the Consent Form for Publication in a PLOS Journal (http://journals.plos.org/plosone/s/file?id=8ce6/plos-consent-form-english.pdf). The signed consent form should not be submitted with the manuscript, but should be securely filed in the individual's case notes. Please amend the methods section and ethics statement of the manuscript to explicitly state that the patient/participant has provided consent for publication: “The individual in this manuscript has given written informed consent (as outlined in PLOS consent form) to publish these case details”.

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Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

1. Is the manuscript technically sound, and do the data support the conclusions?

The manuscript must describe a technically sound piece of scientific research with data that supports the conclusions. Experiments must have been conducted rigorously, with appropriate controls, replication, and sample sizes. The conclusions must be drawn appropriately based on the data presented.

Reviewer #1: Yes

Reviewer #2: Partly

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2. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #1: Yes

Reviewer #2: No

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3. Have the authors made all data underlying the findings in their manuscript fully available?

The PLOS Data policy requires authors to make all data underlying the findings described in their manuscript fully available without restriction, with rare exception (please refer to the Data Availability Statement in the manuscript PDF file). The data should be provided as part of the manuscript or its supporting information, or deposited to a public repository. For example, in addition to summary statistics, the data points behind means, medians and variance measures should be available. If there are restrictions on publicly sharing data—e.g. participant privacy or use of data from a third party—those must be specified.

Reviewer #1: Yes

Reviewer #2: Yes

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4. Is the manuscript presented in an intelligible fashion and written in standard English?

PLOS ONE does not copyedit accepted manuscripts, so the language in submitted articles must be clear, correct, and unambiguous. Any typographical or grammatical errors should be corrected at revision, so please note any specific errors here.

Reviewer #1: Yes

Reviewer #2: Yes

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5. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #1: The paper presents very interesting results dealing with the interactions between sleep posture and chronic spinal pain with practical clinical perspectives. The variables under study and the experimental design seem to be appropriate.

Nevertheless, I found the abuse of acronyms makes the read of the paper quite difficult.

Please explicit the acronyms more regularly (once by paragraph for example) so that the reader does not have to go back to the beginning to verify the sense of the numerous acronyms used in this paper.

Reviewer #2: This paper looks into the association between sleep postures, sleep quality and spinal complaints. Although the study design doe not allow a cause and effect analysis, authors suggest that sleep postures affect spinal complaints. From the introduction I understand that both directions of influence are possible though (refs 4, 11-14) .

Here are my comments and suggestions:

Throughout the paper, authors are describing non-significant findings (p>.05) in a confusing way, i.e. using words as ‘more’, ‘poorer’, ‘greater’. This should be avoided, also when given the exact p value, but especially in the discussion and conclusion where these values do not accompany such statements. In case authors have reason to belief to be dealing with ‘false negative’ results (type II errors), this should be addressed in the discussion.

Line 68: suggestion to specify sleep postures. ‘undesirable’ or something likewise?

Line 90: although I am not a native English speaker, ‘greater than’ 46 years old, should be better ‘older than’ or ‘more than’?

Lines 103 - 109 : why not link postures videoed with complaints that same morning after. Participants in the asymptomatic group also had occasionally symptoms...

Line 120: ‘Higher scores indicated increased symptoms for pain, stiffness and bothersomeness and better quality of sleep.’ Is this correct? Lower quality of sleep?

Line 151-154: is this sentence grammatically correct?

Figure 1: suggestion to indicate in this figure how these postures are believed to result in more or less stress on different parts of the spine.

Line 171-173: why would the position of the head have no influence on the spinal load (torsion?), considering start of the spine at C1?

Line 182-184: why didn't you use these data to make groups?

Line 187: A priori calculated sample size was not reached. This is also less relevant than the power actually obtained with the final dataset. You should report these. See also remark on type 2 errors.

Line 216: ‘achieving a normal distribution was difficult’ is a strange way of putting it. Should be rephrased.

Table 2: suggestion to first explain a and b in the legend.

Line 302: indeed, this may be. You can use SPSS to see how much power you have, and this should be reported. Also reporting effect sizes may be useful especially when p < 0.05.

Line 303: indeed, as suggested above, you can use another approach based on your own observations to allocate into groups.

Lnie 340-351: this info should not be in the conclusion, but rather in the introduction or integrated in the discussion. A conclusion should be concisely answering your research questions.

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6. PLOS authors have the option to publish the peer review history of their article (what does this mean?). If published, this will include your full peer review and any attached files.

If you choose “no”, your identity will remain anonymous but your review may still be made public.

Do you want your identity to be public for this peer review? For information about this choice, including consent withdrawal, please see our Privacy Policy.

Reviewer #1: Yes: Dr. Olivier Coste (MD,PhD)

Reviewer #2: No

[NOTE: If reviewer comments were submitted as an attachment file, they will be attached to this email and accessible via the submission site. Please log into your account, locate the manuscript record, and check for the action link "View Attachments". If this link does not appear, there are no attachment files.]

While revising your submission, please upload your figure files to the Preflight Analysis and Conversion Engine (PACE) digital diagnostic tool, https://pacev2.apexcovantage.com/. PACE helps ensure that figures meet PLOS requirements. To use PACE, you must first register as a user. Registration is free. Then, login and navigate to the UPLOAD tab, where you will find detailed instructions on how to use the tool. If you encounter any issues or have any questions when using PACE, please email PLOS at figures@plos.org. Please note that Supporting Information files do not need this step.

10 Oct 2021

Responses to Reviewers

Comments to the Author

1. Is the manuscript technically sound, and do the data support the conclusions?

The manuscript must describe a technically sound piece of scientific research with data that supports the conclusions. Experiments must have been conducted rigorously, with appropriate controls, replication, and sample sizes. The conclusions must be drawn appropriately based on the data presented.

Reviewer #1: Yes

Reviewer #2: Partly

2. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #1: Yes

Reviewer #2: No

3. Have the authors made all data underlying the findings in their manuscript fully available?

The PLOS Data policy requires authors to make all data underlying the findings described in their manuscript fully available without restriction, with rare exception (please refer to the Data Availability Statement in the manuscript PDF file). The data should be provided as part of the manuscript or its supporting information, or deposited to a public repository. For example, in addition to summary statistics, the data points behind means, medians and variance measures should be available. If there are restrictions on publicly sharing data—e.g. participant privacy or use of data from a third party—those must be specified.

Reviewer #1: Yes

Reviewer #2: Yes

4. Is the manuscript presented in an intelligible fashion and written in standard English?

PLOS ONE does not copyedit accepted manuscripts, so the language in submitted articles must be clear, correct, and unambiguous. Any typographical or grammatical errors should be corrected at revision, so please note any specific errors here.

Reviewer #1: Yes

Reviewer #2: Yes

5. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters).

Reviewer #1: The paper presents very interesting results dealing with the interactions between sleep posture and chronic spinal pain with practical clinical perspectives. The variables under study and the experimental design seem to be appropriate.

Nevertheless, I found the abuse of acronyms makes the read of the paper quite difficult.

Please explicit the acronyms more regularly (once by paragraph for example) so that the reader does not have to go back to the beginning to verify the sense of the numerous acronyms used in this paper.

Author’s Response: Thank you Reviewer #1 for taking the time to provide your valuable suggestions to improve the content and readability of our manuscript. As recommended, we have substantially reduced the number of abbreviations in the manuscript.

Reviewer #2: This paper looks into the association between sleep postures, sleep quality and spinal complaints. Although the study design does not allow a cause and effect analysis, authors suggest that sleep postures affect spinal complaints. From the introduction I understand that both directions of influence are possible though (refs 4, 11-14) .

Author’s Response: Thank you Reviewer #2 for your contribution to the improvement of our manuscript. We agree that our findings should only be interpreted as associations and cannot be used to determine cause and effect. This has been stated explicitly in the revised version.

Here are my comments and suggestions:

Throughout the paper, authors are describing non-significant findings (p>.05) in a confusing way, i.e. using words as ‘more’, ‘poorer’, ‘greater’. This should be avoided.

also when given the exact p value, but especially in the discussion and conclusion where these values do not accompany such statements.

Author’s Response: Use of words that described non-significant differences have been removed from the original Abstract, Results and Discussion sections.

In case authors have reason to belief to be dealing with ‘false negative’ results (type II errors), this should be addressed in the discussion.

Author’s Response: The authors believe it possible a Type II error may have occurred when examining differences in the between the Control and Lumbar groups in combined provocative sleep posture and sleep quality. This possibility has been added (line 283), along with the already stated possible influence of recruitment criteria on lines 324 to 329.

Line 68: suggestion to specify sleep postures. ‘undesirable’ or something likewise?

Author’s Response: In line 27 we have added the word undesirable to associate it with the phrase provocative sleep postures. At line 60 The words ‘flexion or rotation’ have been added into the text to clarify the applied loads from the related references.

Line 90: although I am not a native English speaker, ‘greater than’ 46 years old, should be better ‘older than’ or ‘more than’?’

Author’s Response: The word ‘older’ has been substituted for ‘greater in line 84. To match the phrasing, the word ‘younger’ has also been substituted for ‘less’ at line 83.

Lines 103 - 109 : why not link postures videoed with complaints that same morning after. Participants in the asymptomatic group also had occasionally symptoms.

Author’s Response: Linking morning symptoms with overnight postures is an interesting idea, however our data were not collected with that in mind. Morning data were simply numerical rating scales (NRS) of the severity of pain, stiffness and bothersomeness with no indication of the location or more detailed nature of the symptoms. The morning symptom data were collected to describe the groups in general terms, provide some verification of the group allocations, and to assess whether the nights the sleep data were collected were representative for the participants generally. For the latter, data from the nights of collection were compared to ‘Prior 2 weeks symptoms’ recorded at recruitment

Line 120: ‘Higher scores indicated increased symptoms for pain, stiffness and bothersomeness and better quality of sleep.’ Is this correct? Lower quality of sleep?

Author’s Response: That is correct. Higher scores represented a higher quality of sleep. Line 115 has been modified to clarify this relationship.

Line 151-154: is this sentence grammatically correct?

Author’s Response: The complete paragraph has been rewritten and simplified.

Figure 1: suggestion to indicate in this figure how these postures are believed to result in more or less stress on different parts of the spine.

Author’s Response: The following text has been added after the title “Sustained postures like rotation, have been identified as causing tissue microdamage and muscle spasms”

Line 171-173: why would the position of the head have no influence on the spinal load (torsion?), considering start of the spine at C1?

Author’s Response: It is correct that this would create a degree of cervical torsion, however we decided not to focus on this for several reasons.

1. We had learnt from our pilot study, that when participants slept in supine, the amount of rotation was < 45 degrees and changed frequently, with participants often rotating their head from left to right several times often only for a few seconds, before becoming static or undertaking a major posture shift.

2. The neutral zone of cervical range is considerably larger than the thoracic or lumbar spine, meaning that to reach an end range and cause tissue strain, would most likely require a greater range than 45 degrees.

3. Most other researchers only considered supine, prone and side lying in regards to posture and

4. We believed the plausible spinal loads would be much larger in association with leg positionings, due to the greater weight of the lower limbs.

Line 182-184: why didn't you use these data to make groups?

Author’s Response: Our research questions, groups and the associated exclusion/ inclusion criteria were derived from clinical knowledge. The research design was based on this clinical knowledge and the protocol was registered before data collection commenced. Once the study commenced the registered procedure was followed.

Line 187: A priori calculated sample size was not reached. This is also less relevant than the power actually obtained with the final dataset. You should report these. See also remark on type 2 errors.

Author’s Response: The a priori calculated sample size was not reached and this was reported in our updated clinical trials register. This information has now also been added in Lines 193-4. With regards to not finding a significant result comparing the Lumbar and Control groups in regards to sleep posture, we make mention of a possible type II error in line 325.

Line 216: ‘achieving a normal distribution was difficult’ is a strange way of putting it. Should be rephrased.

Author’s Response: Thank you for noting this. As the majority of the participants in the control group were symptom free the symptom variables were frequently not normally distributed and attempts to transform the data distribution were rarely successful. The sentence has been rephrased on line 217 and now reads “Achieving a normal distribution was not possible for most Control group variables, particularly patient reported outcome variables, as most participants in this group reported low or no symptom levels”.

Table 2: suggestion to first explain a and b in the legend.

Author Response “Data are: a Mean (Standard Deviation) or b Median (Interquartile Range)” has been added to the title of Tables 2 and 3.

Line 302: indeed, this may be. You can use SPSS to see how much power you have, and this should be reported. Also reporting effect sizes may be useful especially when p < 0.05.

Author’s Response: It is true that post hoc power calculations could be performed, however, inclusion of post-hoc power calculations is controversial and we have elected not to make this change. Zhang et al., (2019) argue that they are conceptually flawed and may be analytically misleading. Moreover, presentation of post-hoc power is complicated by the non-normal distribution of much of our data and the consequent need to use non-parametric statistics. As an alternative approach we have talked about the plausibility of the data in terms of patterns present across multiple variables.

Line 303: indeed, as suggested above, you can use another approach based on your own observations to allocate into groups.

Author’s Response: As noted in our Author’s response above, group characteristics and inclusion criteria were created based on prior clinical experience. The Methodology for this study was reported as part of our clinical trials registration, before we collected any data and we subsequently followed the methodology outlined in the clinical trials registration. We are reluctant to change our methods after examining the data.

Line 340-351: this info should not be in the conclusion, but rather in the introduction or integrated in the discussion. A conclusion should be concisely answering your research questions.

Author’s Response: the conclusion has been rewritten (now lines 357 to 363). Extraneous information has been removed and incorporated where appropriate in the Introduction and Discussion sections.

6. PLOS authors have the option to publish the peer review history of their article (what does this mean?). If published, this will include your full peer review and any attached files.

If you choose “no”, your identity will remain anonymous but your review may still be made public.

Do you want your identity to be public for this peer review? For information about this choice, including consent withdrawal, please see our Privacy Policy.

Reviewer #1: Yes: Dr. Olivier Coste (MD,PhD)

Reviewer #2: No.

While revising your submission, please upload your figure files to the Preflight Analysis and Conversion Engine (PACE) digital diagnostic tool, https://pacev2.apexcovantage.com/. PACE helps ensure that figures meet PLOS requirements. To use PACE, you must first register as a user. Registration is free. Then, login and navigate to the UPLOAD tab, where you will find detailed instructions on how to use the tool. If you encounter any issues or have any questions when using PACE, please email PLOS at figures@plos.org. Please note that Supporting Information files do not need this step.

Author’s Response: The original figure file has been uploaded to PACE and corrected according to the PLOS figure specifications.

Submitted filename: Response to reviewers.docx

Click here for additional data file.

15 Nov 2021

Examining relationships between sleep posture, waking spinal symptoms and quality of sleep: A cross sectional study

PONE-D-20-35066R1

Dear Dr. Cary,

We’re pleased to inform you that your manuscript has been judged scientifically suitable for publication and will be formally accepted for publication once it meets all outstanding technical requirements.

Within one week, you’ll receive an e-mail detailing the required amendments. When these have been addressed, you’ll receive a formal acceptance letter and your manuscript will be scheduled for publication.

An invoice for payment will follow shortly after the formal acceptance. To ensure an efficient process, please log into Editorial Manager at http://www.editorialmanager.com/pone/, click the 'Update My Information' link at the top of the page, and double check that your user information is up-to-date. If you have any billing related questions, please contact our Author Billing department directly at authorbilling@plos.org.

If your institution or institutions have a press office, please notify them about your upcoming paper to help maximize its impact. If they’ll be preparing press materials, please inform our press team as soon as possible -- no later than 48 hours after receiving the formal acceptance. Your manuscript will remain under strict press embargo until 2 pm Eastern Time on the date of publication. For more information, please contact onepress@plos.org.

Kind regards,

Matias Noll, Ph.D

Academic Editor

PLOS ONE

Additional Editor Comments (optional):

Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

1. If the authors have adequately addressed your comments raised in a previous round of review and you feel that this manuscript is now acceptable for publication, you may indicate that here to bypass the “Comments to the Author” section, enter your conflict of interest statement in the “Confidential to Editor” section, and submit your "Accept" recommendation.

Reviewer #1: All comments have been addressed

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2. Is the manuscript technically sound, and do the data support the conclusions?

The manuscript must describe a technically sound piece of scientific research with data that supports the conclusions. Experiments must have been conducted rigorously, with appropriate controls, replication, and sample sizes. The conclusions must be drawn appropriately based on the data presented.

Reviewer #1: Yes

**********

3. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #1: Yes

**********

4. Have the authors made all data underlying the findings in their manuscript fully available?

The PLOS Data policy requires authors to make all data underlying the findings described in their manuscript fully available without restriction, with rare exception (please refer to the Data Availability Statement in the manuscript PDF file). The data should be provided as part of the manuscript or its supporting information, or deposited to a public repository. For example, in addition to summary statistics, the data points behind means, medians and variance measures should be available. If there are restrictions on publicly sharing data—e.g. participant privacy or use of data from a third party—those must be specified.

Reviewer #1: Yes

**********

5. Is the manuscript presented in an intelligible fashion and written in standard English?

PLOS ONE does not copyedit accepted manuscripts, so the language in submitted articles must be clear, correct, and unambiguous. Any typographical or grammatical errors should be corrected at revision, so please note any specific errors here.

Reviewer #1: Yes

**********

6. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #1: The authors have taken the remarks of the reviewers very carefully so that the revised paper has been widely improved and can be published without further major modifications now.

**********

7. PLOS authors have the option to publish the peer review history of their article (what does this mean?). If published, this will include your full peer review and any attached files.

If you choose “no”, your identity will remain anonymous but your review may still be made public.

Do you want your identity to be public for this peer review? For information about this choice, including consent withdrawal, please see our Privacy Policy.

Reviewer #1: Yes: Dr Olivier Coste (MD,PhD)

17 Nov 2021

PONE-D-20-35066R1

Examining relationships between sleep posture, waking spinal symptoms and quality of sleep: A cross sectional study

Dear Dr. Cary:

I'm pleased to inform you that your manuscript has been deemed suitable for publication in PLOS ONE. Congratulations! Your manuscript is now with our production department.

If your institution or institutions have a press office, please let them know about your upcoming paper now to help maximize its impact. If they'll be preparing press materials, please inform our press team within the next 48 hours. Your manuscript will remain under strict press embargo until 2 pm Eastern Time on the date of publication. For more information please contact onepress@plos.org.

If we can help with anything else, please email us at plosone@plos.org.

Thank you for submitting your work to PLOS ONE and supporting open access.

Kind regards,

PLOS ONE Editorial Office Staff

on behalf of

Dr. Matias Noll

Academic Editor

PLOS ONE