TSH levels in human, which normally peak in the late evening, are augmented by sleep deprivation. Based on prior research, we postulated that TSH secretion is governed by both sleep and circadian processes. However, environmental and behavioral factors known to affect each of those processes were not controlled in prior investigations. Therefore, we evaluated TSH secretory patterns in three different conditions: 1) entrainment to the 24-h day, 2) a constant routine designed to unmask the endogenous component of circadian rhythmicity, and 3) before and after a light-induced phase shift of the circadian timing system. We found that TSH levels rose over the 4-5 h preceding sleep during entrainment, followed by a precipitous drop at sleep onset. When subjects remained awake on a constant routine, TSH levels remained elevated throughout the nighttime hours. Subjects kept awake for 2 consecutive nights on constant routine showed two distinct cycles of nocturnal TSH secretion, despite increasing sleep deprivation. Both the TSH and body temperature rhythms were substantially shifted, by an equivalent amount, in response to three consecutive nightly exposures to bright light. These data demonstrate that both the output of the human circadian pacemaker and the inhibitory effect of sleep contribute to the regulation of TSH secretion. Under normal conditions, the inhibitory effect of sleep on TSH secretion opposes the nocturnal peak in the circadian TSH drive.