The purpose of our study was to understand the relationship between the components of the three-process model of sleepiness regulation (homeostatic, circadian, and sleep inertia) and the thermoregulatory system. This was achieved by comparing the impact of a 40h sleep deprivation versus a 40h multiple nap paradigm (10 cycles with 150/75min -wakefulness/sleep episodes) on distal (DIST) and proximal (PROX) skin temperatures, core body temperature (CBT), melatonin secretion (MEL), subjective sleepiness and nocturnal sleep EEG Slow-Wave Activity (SWA) in 8 healthy young men in a 'controlled posture protocol'. The main finding of the study is that accumulation of sleep pressure increased subjective sleepiness and SWA during the succeeding recovery night, but did not influence the thermoregulatory system as measured by CBT, DIST and PROX. The circadian rhythm of sleepiness (and PROX) was significantly correlated and phase locked with CBT, whereas DIST and MEL were phase advanced (by 113±28min and 130±30min, respectively; both p<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, most pronounced in DIST; after lights on, the converse occurred. The decay in DIST (vasoconstriction) was significantly correlated with the disappearance of sleep inertia. These effects showed minor and non-significant 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.