Circadian rhythms in several species can be phase-shifted by procedures that stimulate locomotor activity ("exercise") during the usual sleep period. The role of arousal or sleep loss, independent of activity, in this effect has not been adequately resolved. We show here, using the sleep deprivation procedure of gentle handling, that comparably large phase shifts (up to 240 min advances) of the rest-activity cycle can be induced in Syrian hamsters by 3 hr of behavioral arousal, with minimal locomotion, beginning 6 hr before the usual active period. Horizontal distance traveled during the deprivation procedure averaged ϳ0.08 km, compared to 2.5 km typical in exercise studies. Hamsters requiring fewer interventions exhibited larger shifts, suggesting that the level or continuity of spontaneous arousal determines shift size. The circadian rhythm of light-induced c-fos expression in the suprachiasmatic nucleus (SCN) was used as a phase marker to further demonstrate that the clock is reset within 1 hr after a 3 hr deprivation. Sleep deprivation mimicked the effects of exercise on basal c-fos expression in two components of the circadian system, suppressing basal Fos immunoreactivity in the SCN, and increasing Fos in the intergeniculate leaflet. Sleep deprivation without exercise in hamsters can rapidly reset the circadian clock and alter gene expression within the circadian system.Key words: circadian rhythms; nonphotic entrainment; c-fos; wheel running; phase shifts; suprachiasmatic nucleus Forty years ago, it was first suggested that an animal's state of arousal or level of locomotor activity might affect properties of its circadian clock (Aschoff, 1960). Convincing evidence followed some 25 years later, when it was demonstrated that running in an activity wheel can alter the period (Yamada et al., 1986) or shift the phase (Reebs and Mrosovsky, 1989) of circadian rhythms in nocturnal rodents. Since then, it has been shown repeatedly, using a range of arousing stimuli applied during the usual rest phase of the circadian sleep-wake cycle, that substantial phase advance shifts (up to 4 hr) are induced if subjects run during or after the stimulus but usually not if running is absent or prevented (Hastings et al., 1998;. Both the magnitude and the direction of these shifts are gated by circadian phase, with maximal phase advance shifts evident when activity is stimulated near the middle of the rest period and small phase delays (Յ1 hr) when activity is stimulated during the latter half of the usual wake phase of the circadian cycle (Bobrzynska and Mrosovsky, 1998).Although these studies implicate high intensity locomotor activity (i.e., exercise) as the behavioral stimulus critical for phase resetting in response to at least some arousing stimuli, the contribution that sleep loss or nonspecific arousal makes to the phase shifting process, independent of locomotion, has not been adequately resolved. Animals that run little after an arousing stimulus may fail to shift because they do not stay awake, whereas the occasional...