Chronotype description: In search of a solid subjective amplitude scale.

The term "subjective circadian amplitude" refers to the range or the distinctness of diurnal variations of arousal, that is, the awareness (or lack thereof) of difference between hyper- and hypo-activation phases, the ability to volitionally modulate one's own psychophysiological state, the strength of morning-evening preferences and flexibility of the rhythm or perceived stability of the circadian phase. The complexity of this construct is the source of difficulties in research and measurement. The psychometric features of distinctness subscales of the Chronotype Questionnaire and the Caen Chronotype Questionnaire are not satisfactory. In search of the solid subjective amplitude (AM) scale, the Rasch analysis was applied to test 12 behavioral descriptors of circadian rhythm distinctness. The results of the Rasch factor analysis showed unidimensionality of the construct. Rating scale diagnostics of the subjective amplitude scale indicated good fit. However, answer category 3 (neutral agreement on the Likert-type, five-point scale) never emerged as modal and step calibrations negated the monotone incrementality of the scale. Rescoring the scale into a four-point category measure yielded satisfactory OUTFIT indices ranging from 0.90 to 1.10. The newly designed AM scale comprised four items referring to small and four to the large amplitude. The four-point answer option was adopted. The data from 234 subjects (53% women; mean age 31.63 ± 12.99 years) were gathered and analyzed. Percent of the total variance explained in Component Analysis (PCA) reached 45.7% (morningness-eveningness (ME) scale - 26.5%, AM scale - 19.2%). There was no correlation between ME and AM scales (Pearsons's simple correlation coefficient r = -0.018). The internal reliability of the AM scale, as measured with Cronbach's alpha coefficient, proved to be satisfactory: 0.72 (for ME scale - 0.86). Item-total correlations in the AM scale ranged from 0.433 to 0.774 and were significant at p < 0.001. Confirmatory factorial analysis of AM scale indicated mediocre fit: chi-square/degree of freedom = 3.00, root mean square error of approximation = 0.09, standardized root mean square residual = 0.08, comparative fit index = 0.87, Tucker-Lewis index = 0.82. However, the results of Rasch analysis showed good fit statistics for all items: OUTFIT mean squares range from 0.63 to 1.34 and INFIT mean square range from 0.64 to 1.40. All observed values were ≤1.4, which confirmed the new scale as being unidimensional.f If to consider the chronotype in the context of the classical Borbely's two-process model of sleep regulation, it may be assumed that ME dimension relates to the tempo of increasing of sleep pressure during the day, that is, it reflects the homeostatic component of the diurnal rhythm of sleepiness. As to the amplitude, it may be supposed that more distinct rhythm (large amplitude) stands for greater vulnerability to the time of day - it resounds the circadian component of the sleep proneness. It seems that distinct diurnal changes of arousal indicate emotional lability and may be associated with emotional responsiveness, which in turn manifests itself in a characteristic style of coping with stressful situations. One may therefore consider the diurnal variability of arousal as a factor increasing individual susceptibility to mood swings and affective disorders.