Vitamin B12 improves circadian rhythm sleep disorders in humans. Since little is known for its underlying mechanism, we investigated sleep and brain temperature (Tbr)- modulatory activity of four different analogs of vitamin B12 by infusing into the third ventricle of freely behaving male rats for a 10-h nocturnal period. Three doses (1, 10 and 100 nmol) of methylcobalamin (methyl-B12) dose-dependently enhanced both rapid-eyemovement (REM) sleep and non-REM sleep during the infusion period and further induced fluctuations of circadian sleep-waking rhythm for two post-infusion days. The time-course pattern of non-REM sleep and REM sleep was interrelated before and during the drug infusion period but dissociated thereafter. The administration of 100 nmol methyl-B12 resulted in a significant reduction of Tbr during the dark period in phase with the enhancement of sleep. Subsequently, significant fluctuations of circadian rhythm of Tbr occurred over two consecutive days independently from sleep-waking rhythm: a nocturnal elevation on the first post-infusion day followed by a diurnal reduction and a nocturnal elevation on the second post-infusion day. Lower doses of methyl-B12 exerted little effect on Tbr. Two analogs, 5’-deoxyadenosylcobalamin and hydroxocobalamin, at 100 nmol induced similar but less effects on sleep, whereas another analog, cyanocobalamin, at 100 nmol was ineffective. These results suggest that vitamin B12 acutely induced an enhancement of sleep by a direct action on the sleep-regulatory mechanisms, being accompanied by a reduction of Tbr, and that the drug further mediated a modulation of circadian rhythm of sleep-wakefulness and Tbr via its nonspecific action on the biological clock.