Context. It is not yet clear whether the diurnal variation in testosterone is regulated by circadian or homeostatic (sleep) influences.

Objective. The present study tested whether testosterone is driven by a "circadian-independent" sleep effect by shifting sleep acutely to daytime in a 24 h sampling regiment.

Design, setting, participants. In the sleep laboratory, 7 healthy young men (aged 22-32 yr) participated in three conditions, habituation (sleep 2300-0700 h), night sleep (2300-0700 h) and day sleep (0700-1500 h), the latter two in a balanced order.

Intervention & main outcome measure. Serum testosterone was, in all conditions, sampled by hourly blood drawing for 24 h, during constant bed-rest.

Results. Mean testosterone levels increased as a log-linear function of time (h), across both sleep periods (b = 4.88, P < 0.001); from 15.3 ± 2.1 to 25.3 ± 2.2 nmol/liter during night sleep and from 17.3 ± 2.1 to 26.4 ± 2.9 nmol/liter during day sleep. Similarly, mean testosterone levels decreased with time (log-linear) awake (b = -1.80, P < 0.001). There was also evidence of a weak circadian component (acrophase ranging between 0651-0924 h) and an increase with time in the laboratory. Moreover, all these effects, except for the increase during sleep, differed significantly between individuals.

Conclusion. In conclusion, testosterone increased during sleep and fell during waking, whereas circadian effects seemed marginal. Individual differences were pronounced.