Influence of inter-individual differences in circadian flexibility on subjective and objective sleepiness measures during 28-hours of sleep deprivation.

When evaluating sleepiness induced safety consequences it can be important to take into account inter-individual differences in the manifestation of sleepiness. Di Milia and colleague's [1] studied retrospective alertness in subjects with a flexible versus rigid circadian system. They found significant differences in alertness between flexible and rigid types between 04PM and 10PM. The current study explores the influence of individual differences in the flexibility of the circadian system on sleepiness profiles, in response to a lack of sleep. Fifteen participants enrolled in a 28 h total sleep deprivation study, of which 4 men and 11 women. Participants were selected on the basis of their circadian phenotype, assessed by the factor "Flexibility" of the Circadian Type Index (CTI) [1] and were divided in two groups: rigid types (CTI flexibility score 27, 75th percentile; mean age=22.25, SD=6.50). Inclusion criteria were: age between 18 and 39 years old, good physical health (SF-20) [2], [3], no psychiatric problems (MINI) [4], habitual sleep duration between 6,5 and 8,5 hours, good sleep quality (Pittsburg Sleep Quality Index; PSQI) [5], no excessive daytime sleepiness (Epworth Sleepiness Scale; ESS) [6], no extreme morning-evening types [7], no previous detrimental neuropsychiatric reactions to sleep deprivation, no shift work, no current medical treatment or use of medicines or drugs (excluding oral contraceptives) influencing sleep. The Karolinska Sleepiness Scale (KSS) [8], the Psychomotor Vigilance Task (PVT) [9] and the Alpha Attenuation test (AAT) [10], were used to asses subjective, performance based and physiological sleepiness. Results showed an increase of sleepiness under conditions of extended wakefulness. A sudden increase of sleepiness occurred when the biological night time started, with a rather stable level of sleepiness during the rest of the sleep deprivation period. Inter-individual differences in the behavioral flexibility of the circadian system were reflected in differential sleepiness profiles, suggesting that the circadian rhythm parameter of flexibility can be pivotal in evaluating sleepiness and sleepiness induced safety consequences. Flexible subjects displayed higher levels of sleepiness during the biological day, together with lower levels of subjective and performance based sleepiness during the biological night. In line with the previous results of Di Milia and colleague's [1] our findings suggest that the circadian rhythm parameter of flexibility may be an important factor in evaluating the tolerance to shift work and to other conditions associated with functioning during nighttime.