Sleep patterns and the risk for ADHD: a review.

Attention-deficit/hyperactivity disorder (ADHD) is often associated with comorbid sleep disturbances. Sleep disturbances may be a risk factor for development of the disorder, a symptom of the disorder, or a comorbid condition affected by a similar psychopathology. Various studies have examined the impact of sleep deprivation on the presence/exacerbation of ADHD symptomology, as well as longitudinal and concurrent associations between different sleep disturbances and ADHD, yet the notion of sleep disturbances as a predecessor to ADHD remains unclear. As such, this review examines the evidence for sleep disturbances as a risk factor for the development of ADHD, as well as the mechanisms underlying the association between sleep patterns and ADHD. Additionally, clinical implications regarding the comorbid nature of sleep disturbances and ADHD will be considered.

Introduction

Attention-deficit/hyperactivity disorder (ADHD) is one of the most commonly diagnosed childhood disorders, affecting approximately 3%–5% of school-aged children1 and enduring throughout adolescence and adulthood. A diagnosis of ADHD is dependent on developmentally inappropriate symptoms of inattention, hyperactivity, and/or impulsivity, with onset before the age of 7 years, and impaired functioning in two or more settings.2 Sleep difficulties are often comorbid with ADHD.3 In fact, 25%–50% of children and adolescents with ADHD also report sleep disturbances.4 Despite the high comorbidity between sleep disturbances and psychopathology, it is unclear if sleep disturbance is a cause, a consequence, or a comorbid problem that is associated with such disturbances. Previous research has examined the complexity of the relationship between sleep disturbances and ADHD and a bidirectional association has been suggested,5 such that sleep contributes to increased manifestation of ADHD symptoms (eg, inattention, hyperactivity, impulsivity) and ADHD contributes to worsened sleep.6 To what extent sleep problems are a risk for development of ADHD versus the outcome of the psychological condition is unclear, however. Thus, this review will examine (1) evidence for various sleep aspects as risk factors for both exacerbation and development of ADHD; (2) mechanisms underlying the relationship between sleep disturbances and ADHD; and (3) clinical implications of the interplay between sleep and ADHD.

Methods

A literature search was performed using PubMed, Google Scholar, and PsycINFO. The following keywords were used in various combinations: sleep, psychopathology, ADHD, attention deficit hyperactivity disorder, risk factor, predictor, development, sleep restriction, sleep deprivation, sleep patterns, insomnia, and sleep problems. Reference sections of appropriate articles were also used to obtain other relevant articles. Articles that directly assessed the variables of interest (sleep and ADHD) were kept and precedence was given to studies showing a directional relationship between sleep and ADHD. Nine articles concerning the longitudinal association between sleep disturbances and ADHD, three articles pertaining to experimental sleep deprivation and ADHD, nine correlational studies utilizing regression to assess sleep disturbances as a risk for ADHD, and three treatment outcome studies involving sleep disorder treatment and ADHD outcomes, were reviewed.

Although definitions of sleep terminology are inconsistent amongst various studies, in this review, sleep deprivation refers to total sleep deprivation, that is, the purposeful elimination of sleep during at least a 24 hour period7 and/or partial sleep deprivation (also known as sleep restriction), that is, the purposeful elimination of sleep from one’s needed amount for optimal performance; and sleep disturbances refers to diagnosable sleep disorders, such as sleep onset insomnia, parasomnias and other sleep disorders, as well as dysfunctions of sleep architecture and continuity.8

Sleep deprivation as a risk factor for ADHD symptoms

Although some studies fail to find an association between sleep restriction and ADHD symptomology,9 the majority of studies with typically developing children and children with ADHD have demonstrated that sleep deprivation can result in deficits in neurobehavioral functioning that resembles or exacerbates ADHD symptoms (see Table 1 for a summary of studies). For example, Sadeh et al10 and Gruber et al11 assessed the effect of cumulative sleep deprivation on neurobehavioral functioning of both typically developing children11,12 and children with ADHD.12 In both studies, it was found that sleep deprivation resulted in a significant decrease in performance on the continuous performance task, a neurobehavioral task requiring sustained attention and behavioral inhibition that is commonly used in the evaluation of inattention and impulsivity in children with ADHD. Furthermore, in the study by Gruber et al12 performance of the ADHD group deteriorated from subclinical levels of inattention to clinical levels of inattention following sleep deprivation, emphasizing the negative impact on ADHD symptomology.

Table 1

Studies examining the risk of developing ADHD based on sleep restriction, sleep disturbances and sleep disorders

Author	Sample size	Age at first assessment	Sleep measures	Diagnostic measures	Timeframe	Main findings
Gregory and O’Connor16	490	4 years	Single sleep problem scale	Attention and aggression subscales of the CBCL66	11 years	Sleep problems at 4 years of age predicted attention and aggression problems at age 15; behavior problems, with the exception of inattention, did not predict sleep problems.
Touchette et al15	1492	2.5 years	Self-administered questionnaire for the mother	Interviewer completed computerized questionnaire	3.5 years	Risk for HI was 3.2 times higher for short increasing sleepers compared with 11 hour sleepers.
O’Callaghan et al17	4202	6 months/2–4 years	Maternal report of child sleep problems (ie, quality of sleep, sleeplessness)	Attention subscale of the CBCL66	13.5 years	Sleep problems occurring ‘sometimes’ in boys predicted attention problems at age 5; irregular sleep habits occurring ‘often’ from the ages of 2 and 4 in both sexes predicted attention problems at 5 years.
Friedman et al18	568	4 years	CBCL66	Laboratory assessment of inhibiting, updating, and shifting abilities	12 years	Sleep problems at 4 years did not predict executive functioning in adolescence; individuals with sleep problems that decreased more throughout the study demonstrated greater executive functioning in adolescence.
Thunström et al62	54	6–12 months	Five open ended questions about child’s sleep pattern according to ICSD classifications67	DSM-IV criteria2 assessment by multidisciplinary team	5 years	Family issues, behavior problems at bedtime, and long sleep onset latency during infancy were associated with an ADHD diagnosis at age 5.5 years old.
Chervin et al20	229	2–13 years	Snoring, restless leg, and sleepiness items of pediatric sleep questionnaire53	Hyperactivity subscale of the Conner’s parent rating scale68	Until 4 years after initial assessment	Hyperactivity at follow-up was associated with: baseline habitual snoring, loud snoring, composite score on snoring, sleepiness, or sleep-disordered breathing.
Chervin et al21	143	2–18 years	Snoring, restless leg, and sleepiness items of pediatric sleep questionnaire53	Scale for inattention and hyperactivity derived from the DSM-IV2	Concurrent	Snoring was more frequent among children with ADHD. Snoring scores were associated with increased inattention and hyperactivity.
Gottlieb et al63	3019	5 years	Parent completed survey regarding symptoms of sleep disordered breathing	Parent reported responses to questions validated with the Conners’ parent rating scale – revised65	Concurrent	Children with SDB symptoms had more daytime sleepiness, inattention, hyperactivity compared with children without symptoms of SDB.
Paavonen et al55	280	7–8 years	Actigraphy, parent completed sleep disturbance scale for children70	Parent completed attention-deficit/hyperactivity disorder rating scale IV71	Concurrent	Short sleep duration was associated with hyperactivity and impulsivity, while sleep disturbances were associated with hyperactivity, impulsivity, and attention problems.
Pesonen et al60	470	8 years	Actigraphy assessment for 7 nights	Parent completion of the CBCL66	Concurrent	Short sleep duration was significantly related to ADHD problems as rated by mothers; short sleep duration was significantly related to more externalizing symptoms as rated by fathers.
Owens et al64	235	3–18 years	CSHQ54; pediatric sleep disorder clinic evaluation	CBCL66	Concurrent	Short sleep was associated with increased externalizing scores. Reporting at least one additional sleep disorder diagnosis was the greatest predictor of adverse outcomes on the CBCL.
O’Brien et al61	5728	5–7 years	Parent completed sleep survey; Polysomnography	Parent completed questionnaire about hyperactivity; Conner’s Parent Rating Scale72 and the CBCL66	Concurrent	SDB symptoms were significantly associated with mild, but not severe, ADHD symptoms as measured by the Conner’s parent rating scale.
Konofal et al24	32	5–9 years	NIH-specific criteria73 for restless leg syndrome	Conners’ parent rating scale72	Concurrent	There was a trend towards higher ADHD symptomology in children with ADHD and restless leg syndrome than in children with ADHD only.
Kass et al65	148	Mean = 22.7 years	Epworth sleepiness scale74; Athens insomnia scale75	Adult behavior checklist76	Concurrent	Sleep scores were associated with attention deficit and hyperactivity subscales of the adult behavior checklist.

Abbreviations: ADHD, attention-deficit/hyperactivity disorder; CBCL, child behavior checklist; CSHQ, children’s sleep habits questionnaire; DSM, Diagnostic and Statistical Manual of Mental Disorders; HI, hyperactivity-impulsivity; ICSD, International Classification of Sleep Disorders; NIH, National Institutes of Health; SDB, sleep-disordered breathing.

Further support for the impact of sleep deprivation on inattention has been exhibited through findings in studies utilizing teacher reports of ADHD symptoms. For instance, using an experimental sleep restriction paradigm, Fallone and colleagues12 found that following sleep restriction, observers reported increased attention difficulties.12 Gruber and colleagues13 have also recently shown that teachers of typically developing school-aged children, who were blind to their students’ sleep status, reported increased cognitive problems and inattention after sleep restriction confirmed by polysomnography. In another longitudinal study by Touchette and colleagues,14 sleep duration at ages 2.5, 3.5, 4, 5, and 6 years old predicted parent reported hyperactivity-impulsivity at age 6. Additionally, the risk for hyperactivity-impulsivity was 3.2 times greater for children who had shorter sleep duration at 41 months old as compared to those receiving adequate sleep.

Collectively, these findings suggest that cumulative sleep deprivation is associated with increased ADHD symptoms of inattention. However, sleep deprivation is not the only problem encountered by children with ADHD and it is important to understand the impact that sleep disturbances may have on ADHD symptomology.

Sleep disturbances as a risk factor for ADHD

Sleep disturbances, such as fragmented sleep, bedtime resistance, and sleep onset insomnia in early childhood, have been found to be associated with increased likelihood of developing ADHD symptoms later on. For instance, Gregory and O’Connor15 examined the concurrent and predictive associations between sleep and behavioral problems in children over an 11-year period and found that sleep disturbances at age 4 predicted attention and aggression problems at age 15. Another study16 revealed similar associations, with sleep problems between the ages of 2–4 predicting attention deficits at age 5. These studies suggest that sleep disturbances in early childhood may be an initial symptom of later ADHD, or may be a causal factor in the development of future ADHD symptoms.

Not just the presence of sleep disturbance, but also the stability of sleep disturbance over time appears to be important in determining to what extent it is a risk factor for behavioral disorders such as ADHD. Friedman and colleagues17 conducted a longitudinal study following children from the age of 4, 5, 7, and 9–16 years old. Results revealed that individuals whose sleep disturbances (ie, bedwetting) decreased over time showed better executive control in adolescence than those whose sleep disturbances remained relatively stable. These findings suggest that the presence of stable sleep disturbances over childhood is a significant risk factor for the development of subsequent ADHD.

It is not unusual for a diagnosis of ADHD to be accompanied by comorbid primary sleep disorders, such as sleep-disordered breathing (SDB) and restless legs syndrome (RLS). Various studies have examined the association between these disorders and ADHD symptomology and have found that primary sleep disorders can exacerbate ADHD symptoms.

SDB

SDB, including frequent snoring and obstructive sleep apnea, has been associated with increased levels of hyperactivity and inattention.18 For example, Chervin and colleagues19 found that children demonstrating elevated scores on the snoring index of a SDB measure were at increased risk for developing hyperactivity symptoms 4 years later. In another study,20 it was found that untreated SDB was related to a higher likelihood of being diagnosed with ADHD. Furthermore, research examining children with comorbid obstructive sleep apnea and ADHD21,22 has demonstrated that treatment of the sleep disorder by adenotonsillectomy surgery results in significant improvements in attention and reduced hyperactivity.

RLS

There is evidence of a positive association between a diagnosis of RLS and severity of ADHD symptoms. For example, Konofal and colleagues23 investigated the impact of RLS on ADHD severity in 5–8 year old children and found that ADHD symptoms were more severe in children with both ADHD and RLS than ADHD alone. It was proposed that the sleep fragmentation involved with RLS might exacerbate symptoms of attention and hyperactivity associated with ADHD. Taken together, these findings support the notion that sleep problems can play a substantial role in contributing to ADHD symptoms and suggest an important link between sleep pathophysiology and the pathophysiology of ADHD.

Developmental considerations

It was originally believed that ADHD symptoms tended to wane in adolescence. This may account for the fact that, in comparison to the abundance of research on ADHD in pediatric populations, the research on ADHD in adult populations is quite sparse. However, it is known that ADHD is a lifelong disorder affecting individuals from childhood to adulthood. Studies24–28 show that, similar to the sleep complaints demonstrated by children and adolescents with ADHD, sleep problems are also common in adults with the disorder, as evidenced by high rates of self-reported sleep problems in up to 83% of adults assessed for ADHD. Data on sleep efficiency in adults with ADHD are inconsistent, with one PSG study depicting no differences as compared with normal controls,28 whereas another found reduced sleep efficiency, an elevated number of awakenings, a higher percentage of wakefulness after sleep onset, and a reduction in the percentage of rapid eye movement (REM) sleep in individuals with the disorder.29 In summary, the sleep disturbances characteristics of adults with ADHD seem to resemble those found in children including reduced REM sleep and nocturnal awakenings. This suggests the notion that rather than waning in adolescence, sleep disturbances in individuals with ADHD persist throughout the lifespan.

Common underlying pathophysiology between sleep problems and ADHD

Several common underlying mechanisms have been suggested to account for the link between sleep disturbances and symptoms of ADHD. In particular, prefrontal cortex (PFC) functions, dopamine, and the circadian system have been suggested as potential links between the sleep-wake system and ADHD.

PFC

The PFC is implicated in the regulation of sleep and wakefulness, as well as in carrying out executive functions.30 Executive functions consist of behavioral inhibition, attentional tasks, set-shifting, working memory, analysis/synthesis, and contextual memory,31 functions which are impaired in individuals with ADHD. Various sleep deprivation32,33 studies have found that the functions most affected are those controlled by the PFC – namely, the executive functions. This process has been found to occur through the disruption of cellular homeostasis in the PFC.31 As such, underlying deficits in the PFC as a result of sleep disruption can lead to executive functioning deficits characteristic of individuals with ADHD.

Dopaminergic system

Dopaminergic activity, particularly in the PFC, is associated with ADHD symptomology34 and is also implicated in the regulation of sleep and waking.35 Dopamine has been found to be important in modulating performance on tasks of executive functions,36 such as attention, which are the areas of greatest impairment in individuals with ADHD.37,38 As well, dopamine levels in the substantia nigra and the ventral tegmental area have been shown to be involved in promoting wakefulness.39 For example, injection of dopamine agonists into rat brains results in more wakefulness and wake-like behaviors,40 showing that higher dopaminergic activity is involved in maintaining wakefulness. Other evidence comes from studies which show that dopamine levels tend to be decreased following sleep restriction and are associated with impaired waking functions.41

Further evidence for dopamine’s role in ADHD and the sleep-wake cycle comes from genetic studies examining catechol-O-methyltransferase (COMT), a gene that encodes for a dopamine-degrading enzyme. It has been suggested that COMT activity contributes to sleep problems in individuals with ADHD because of its role in the metabolism of dopamine.42 The high activity variant of the COMT gene, the valine (Val) allele, results in decreased concentrations of dopamine compared to that of the low activity variant, methionine (Met). When sleep was compared in children with ADHD having the Val-Val or Val-Met polymorphisms to those with the low activity Met-Met polymorphism, it was revealed that children with decreased cortical dopamine had poorer sleep continuity than their counterparts.43 Such research suggests a role of dopamine in the underlying genetic pathophysiology of ADHD and sleep disturbances.

Circadian system

A maladjusted circadian rhythm may also be implicated in the potential pathophysiological association between sleep disturbances and ADHD.44 Indeed, delayed circadian rhythms, as evidenced by a delayed nighttime increase in endogenous melatonin, have been demonstrated in nonmedicated children with ADHD.45 Such a delay in their endogenous circadian system may disrupt the timing of their sleep and wake periods and account for other sleep disturbances. Additionally, deficits in the circadian system may result in increased daytime fatigue and in bedtime refusal,45 potentially mimicking ADHD-like symptoms. In this way, the circadian system may offer another potential pathophysiological explanation for the link between sleep disturbances and ADHD.

Clinical implications

Given that sleep disorders, such as obstructive sleep apnea and RLS, as well as the presence of chronic sleep disturbances and/or sleep deprivation, can result in behaviors that mimic or exacerbate ADHD-symptomology12,46–49 (eg, hyperactivity

and inattention), screening for such conditions is an essential component in the assessment of children with symptoms of inattention or hyperactivity.50 Sleep assessments during the ADHD diagnostic process are feasible in primary care settings through brief screening tools (eg, BEARS51 [B = bedtime issues, E = excessive daytime sleepiness, A = night awakenings, R = regularity and duration of sleep, S = snoring]) and parent report surveys (eg, Pediatric Sleep Questionnaire,52 Child Sleep Habit Questionnaire53). A review of sleep problems should occur during the baseline assessment (ie, when diagnosing ADHD), as well as on an ongoing basis during the management of the disorder.51 Polysomnography should be considered to confirm sleep disorder diagnoses for individuals whose reported symptoms are suggestive of breathing problems or nocturnal leg movements.54 As well, sleep diaries and actigraphy55 are useful tools for evaluating quantity and quality of sleep, as well as assessing variability of sleep patterns in children with ADHD. When sleep problems or disorders are encountered, it is important to treat both the symptoms of ADHD, as well as the sleep problems themselves, as the presence of such sleep problems will likely decrease the efficacy of an intervention aimed solely at improving ADHD symptoms.

Finally, the presence of sleep problems should be considered when prescribing stimulant medications, as these can produce symptoms of insomnia or worsen already existing problems.56,57 As such, individuals with ADHD should be monitored for development of sleep problems following medication administration, as well as having their medications titrated to optimum doses for improving ADHD symptoms, while limiting potential negative effects on sleep.

Methodological considerations

The interpretation of findings obtained in studies regarding sleep and ADHD is challenging given the inconsistent definitions of sleep problems and the measures used to record sleep. The reviewed studies are at times inconsistent with regards to their definition of sleep restriction, disorders, and disturbance and, thus, utilize varying diagnostic criteria when examining sleep problems associated with ADHD. For example, objective (eg, apnea hypopnea index23) and subjective (eg, Pediatric Sleep Questionnaire20) are being used to define sleep disordered breathing. In addition, sleep recording methods, as well as the types and number of sleep para meters measured using these methods, vary between studies. Different studies use different objective actigraphy58,59 and polysomnography14,61 or subjective parental report15–18,20,21,60–64 measures, and, as a result, it is difficult to compare findings between studies. It is, therefore, crucial for future studies to make an attempt to utilize consistent, sleep related terminology and diagnostic criteria when examining the relationship between sleep and ADHD.

Summary and future directions

Sleep deprivation and sleep disturbances are prevalent in children with ADHD and have been shown to exacerbate ADHD symptomology, including inattention, hyperactivity, and impulsivity. Additionally, restoration of proper sleep patterns in children with ADHD has been found to alleviate symptomology, whereas experimental sleep restriction has been shown to worsen such symptoms, suggesting that sleep problems can contribute to the manifestation of ADHD symptoms. Furthermore, longitudinal research suggests that sleep disturbances in early childhood are a risk factor for the subsequent development of ADHD later on in childhood and adolescence. As such, although the relationship between sleep disturbances and ADHD appears to be bidirectional, there is support for the role of sleep disturbances as a predecessor to the disorder. It is, thus, important for healthcare providers to integrate sleep components into ADHD diagnostic and treatment plans. Future research should investigate the relationship between ADHD etiology (eg, ADHD and comorbid psychopathologies, ADHD and comorbid sleep disorders, and ADHD without comorbid sleep disorders) and sleep disturbances, as well as clarify the effects of sleep deprivation on specific ADHD symptomology. Additional longitudinal research is needed to further understand the potential causal relationship between sleep patterns and ADHD.

The association between disrupted sleep patterns and ADHD appears to be modulated by the PFC and the dopaminergic system. Further elucidation of the ways in which these systems relate to both sleep and the presence of psychological disorders can help improve interventions by targeting both the disorder and the sleep disturbances involved.