PERSISTENCE OF THE CIRCADIAN THYROTROPIN RHYTHM UNDER CONSTANT CONDITIONS AND AFTER LIGHT-INDUCED SHIFTS OF CIRCADIAN PHASE

TSH levels in humans, which normally peak in the late evening, are aug mented by sleep deprivation. Based on prior research, we postulated th at TSH secretion is governed by both sleep and circadian processes. Ho wever, environmental and behavioral factors known to affect each of th ose 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 f ound that TSH levels rose over the 4-5 h preceding sleep during entrai nment, followed by a precipitous drop at sleep onset. When subjects re mained awake on a constant routine, TSH levels remained elevated throu ghout the nighttime hours. Subjects kept awake for 2 consecutive night s on constant routine showed two distinct cycles of nocturnal TSH secr etion, despite increasing sleep deprivation. Both the TSH and body tem perature rhythms were substantially shifted, by an equivalent amount, in response to three consecutive nightly exposures to bright light. Th ese data demonstrate that both the output of the human circadian pacem aker and the inhibitory effect of sleep contribute to the regulation o f TSH secretion. Under normal conditions, the inhibitory effect of sle ep on TSH secretion opposes the nocturnal peak in the circadian TSH dr ive.