-The purpose of our study was to understand the relationship between the components of the threeprocess model of sleepiness regulation (homeostatic, circadian, and sleep inertia) and the thermoregulatory system. This was achieved by comparing the impact of a 40-h sleep deprivation vs. a 40-h multiple nap paradigm (10 cycles with 150/75 min wakefulness/sleep episodes) on distal and proximal skin temperatures, core body temperature (CBT), melatonin secretion, subjective sleepiness, and nocturnal sleep EEG slow-wave activity in eight healthy young men in a "controlled posture" protocol. The main finding of the study was that accumulation of sleep pressure increased subjective sleepiness and slow-wave activity during the succeeding recovery night but did not influence the thermoregulatory system as measured by distal, proximal, and CBT. The circadian rhythm of sleepiness (and proximal temperature) was significantly correlated and phase locked with CBT, whereas distal temperature and melatonin secretion were phase advanced (by 113 Ϯ 28 and 130 Ϯ 30 min, respectively; both P Ͻ 0.005). This provides evidence for a primary role of distal vasodilatation in the circadian regulation of CBT and its relationship with sleepiness. Specific thermoregulatory changes occur at lights off and on. After lights off, skin temperatures increased and were most pronounced for distal; after lights on, the converse occurred. The decay in distal temperature (vasoconstriction) was significantly correlated with the disappearance of sleep inertia. These effects showed minor and nonsignificant circadian modulation. In summary, the thermoregulatory system seems to be independent of the sleep homeostat, but the circadian modulation of sleepiness and sleep inertia is clearly associated with thermoregulatory changes. circadian rhythm; skin and core body temperature; electroencephalogram slow-wave activity SLEEPINESS CAN BE DEFINED as a physiological need for sleep with a behavioral measure of the subject's tendency to fall asleep at a certain time (sleep propensity) (3, 25). Sleepiness, and its converse "alertness" (in this study only sleepiness is used), is a regulated and important determinant of psychomotor vigilance and performance (19). Three major processes are considered to be involved in regulating sleepiness: 1) a homeostatic process that is manifested in a growing increase of sleep propensity during the course of wakefulness that is dissipated during sleep (7,14); 2) a circadian process controlled by the circadian pacemaker in the suprachiasmatic nuclei that produces a maximum drive for alertness during the subjective day and a maximum drive for sleepiness during the subjective night (7, 14); and 3) a process of "sleep inertia," which describes the phenomenon of low vigilance on awakening even though sleepiness should be lowest at the end of a sleep episode (24). These three processes represent the compartments of the mathematical three-process-model for sleepiness (2,18).Many studies have shown that the level of EEG slow-wave activity (S...