The rhythm of plasma melatonin originating from the pineal gland and driven by the circadian pacemaker located in the suprachiasmatic nucleus is closely associated with the circadian (approximately 24 h) variation in sleep propensity and sleep spindle activity in humans. We investigated the contribution of melatonin to variation in sleep propensity, structure, duration and EEG activity in a protocol in which sleep was scheduled to begin during the biological day, i.e. when endogenous melatonin concentrations are low. The two 14 day trials were conducted in an environmental scheduling facility. Each trial included two circadian phase assessments, baseline sleep and nine 16 h sleep opportunities (16.00-08.00 h) in near darkness. Eight healthy male volunteers (24.4 +/- 4.4 years) without sleep complaints were recruited, and melatonin (1.5 mg) or placebo was administered at the start of the first eight 16 h sleep opportunities. During melatonin treatment, sleep in the first 8 h of the 16 h sleep opportunities was increased by 2 h. Sleep per 16 h was not significantly different and approached asymptotic values of 8.7 h in both conditions. The percentage of rapid eye movement (REM) sleep was not affected by melatonin, but the percentage of stage 2 sleep and sleep spindle activity increased, and the percentage of stage 3 sleep decreased. During the washout night, the melatonin-induced advance in sleep timing persisted, but was smaller than on the preceding treatment night and was consistent with the advance in the endogenous melatonin rhythm. These data demonstrate robust, direct sleep-facilitating and circadian effects of melatonin without concomitant changes in sleep duration, and support the use of melatonin in the treatment of sleep disorders in which the circadian melatonin rhythm is delayed relative to desired sleep time.