Individual differences in the tolerance to sleep deprivation: the effects of circadian flexibility

Objectives. The current study was designed to investigate the influence of individual differences in the flexibility of the circadian rhythm on sleepiness, during 28 hours of sleep deprivation. Methods. Based on the flexibility of the circadian rhythm, as measured with the Circadian Type Inventory, 17 rigid (mean age 23 years, SD=4.9) and 17 flexible types (mean age 22 years, SD=4.5) were selected. These participants were deprived from sleep for 28 hours, in controlled laboratory conditions. Subjective and performance based sleepiness were determined during the sleep deprivation period by use of the Karolinska Sleepiness Scale (KSS; hourly measures) and the Psychomotor Vigilance Task (PVT; 2 hourly measures), with a focus on reaction speed (RS).Results. An increase in subjective and performance based sleepiness was observed under conditions of prolonged wakefulness (FKSS(5.476,164.285)=52.643, p