Habitual sleep duration varies greatly among individuals. The physiological basis of this variation is unknown. We sought to determine whether individual differences in sleep duration are associated with systematic differences in the duration of the biological night that is programmed by the circadian pacemaker and reflected in the nocturnal interval of circadian rhythms in neuroendocrine function, body temperature, and arousal. Ten young, healthy long sleepers (sleep duration >9 h) and 14 short sleepers (<6 h) were studied under constant environmental conditions and in the absence of sleep. The nocturnal intervals of high plasma melatonin levels, increasing cortisol levels, low body temperature, and increasing sleepiness were longer in long sleepers than in short sleepers. The maxima in cortisol and sleepiness exhibited a close relationship to habitual wake-up time, which occurred approximately 2.5 h later in long sleepers than in short sleepers. It is concluded that the circadian pacemaker programs a longer biological night in long sleepers than in short sleepers. We propose that individual differences in the circadian pacemaker's program may contribute to the variability of sleep duration in the general population. The persistence or inertia of an individual's circadian program, as was evident in constant conditions, may underlie the commonly experienced difficulty of changing habitual sleep duration willfully.