Sleep-promoting and hypothermic effects of orally administered melatonin during the daytime were assessed using a placebo-controlled, double-blind, cross-over design. Following a 7-hour nighttime sleep opportunity, healthy young male subjects (n = 8) were given either a placebo or one of three doses of melatonin (1 mg, 10 mg, and 40 mg) at 1000 hours. Sleep was polygraphically assessed in a 4-hour sleep opportunity from 1200 to 1600 hours. All doses of melatonin significantly shortened the latency to sleep onset. Melatonin also significantly increased total sleep time and decreased wake after sleep onset (WASO). Sleep following melatonin administration contained significantly more stage 2 and less stage 3-4, while stage 1 and rapid eye movement (REM) sleep were unaffected. In addition to the sleep-promoting effects, melatonin completely suppressed the normal diurnal rise of core body temperature. These data suggest that melatonin may be an effective method of promoting sleep for individuals attempting to sleep during their subjective day, such as shiftworkers and individuals rapidly traveling across multiple time zones.