A six-factor structure of individual variation in the tendencies to become sleepy and to sleep at different times of the day.

A multidimensional approach has been previously applied for modeling and assessment of individual differences in the ability to sleep or to stay awake at certain clock hours. More recently the 19 time-point Visuo-verbal Judgment Task (VJT) has been proposed to predict changes in sleepiness level from the morning hours to the next day afternoon. The dimensionality of the VJT has not been explored so far. We applied a structural model of individual variation in sleep-wake behavior and habits for explaining the pattern of relationship between the VJT's dimensions yielded by rotation of principal components with eigenvalue>1. The responses to 19 items of the VJT, 72 items of 6-scale Sleep Wake Pattern Assessment Questionnaire (SWPAQ) and 60 items of 6-scale Sleep-Wake Adaptability Test (SWAT) were collected from 1037 survey participants. Factor analyses yielded 4 factorial dimensions of morning (08:00-11:00), daytime (12:00-20:00), nighttime (21:00-04:00), and after 24 h sleepiness (06:00-12:00 next day). The model was found to be capable to explain the correlations among 4 constructs of the VJT as well as the correlations among previously developed scales of the SWPAQ and SWAT. The results confirmed the predictive power of the model and its applicability for multidimensional assessments in chronobiological and chronopsychological studies.