Evidence has been provided for an influence of nocturnal sleep on the secretion of cortisol, LH, and GH in man. Although nocturnal sleep inhibits cortisol secretion during the first hours, it augments the secretion of LH and GH. To separate the effects of circadian rhythm from those of sleep, the present experiments examined the influence of diurnal sleep on the release of cortisol, LH, and GH in 12 young men. Subjects slept on 2 different occasions. After a night of wakefulness, subjects were assigned to bed at 0800 h the following morning. Lights were turned off either at 1100 or 1500 h to enable sleep. Effects of diurnal sleep were evaluated by comparing blood hormone concentrations during the interval from 1100-1500 h between subjects when sleeping and awake. Comparing hormonal concentrations during the 4 h of sleep after 1100 h with those during the 4-h sleep interval after 1500 h provided evidence for an influence of circadian rhythm on cortisol and LH release. Diurnal sleep, as has been shown for nocturnal sleep, augmented the secretion of LH and GH. However, in contrast to nocturnal sleep, diurnal sleep failed to suppress cortisol release, suggesting that sleep does not inhibit cortisol release at any point of its circadian rhythm, but only within a limited range of entrainment.