Sleep-wake behavior, as well as sleepiness, is regulated by the joint action of an exponentially increasing drive for sleep--sleep homeostasis--and by variations in sleep propensity due to a biological circadian oscillator. However, large inter-individual differences remain. Short and long sleepers have been known to differ in the amount of homeostatic sleep pressure: long sleepers report higher levels of subjective sleepiness after sleep deprivation, whereas short sleepers exhibit no significant increase in sleepiness. The circadian pacemaker's program might be at the origin of the variability in habitual sleep duration in long and short sleepers. Previous studies within our group showed that the dynamics of both processes are similar in experienced sleepiness as in judged sleepiness. The aim of the present investigation was to determine whether habitual short, long and midrange sleepers exhibit similar integration patterns or whether the biological underpinnings of habitual sleep time variability emerge when judging sleepiness. Our results show an additive integration rule for homeostatic and circadian determinants of daytime sleepiness in all groups. However, rescaled functional values (RSFVs) and relative range indices (RRIs) suggest that short sleepers seem to tolerate higher levels of homeostatic sleep pressure in comparison to long sleepers and that in long sleepers, circadian variability plays a less prominent role in functional daytime sleepiness. As sleep-wake behavior is governed by both physiological mechanisms and psychological processes influenced by past experiences related to sleep and wake, these results have implications for sleep-wake-related health and safety. (Contains 5 figures, 5 tables, and 2 footnotes.)