CONSERVATION OF PHOTOPERIOD-RESPONSIVE MECHANISMS IN HUMANS

In animals, circadian pacemakers respond to seasonal changes in day le ngth by making corresponding adjustments in the durations of diurnal a nd nocturnal periods of circadian rhythms; these adjustments mediate e ffects of photoperiod on breeding and other seasonally recurring pheno mena. Little is known about photoperiod responses of human circadian p acemakers. To investigate this question, we recorded and compared circ adian rhythm profiles of 15 individuals after chronic exposures to sho rt (8 h) and long (14 h) nights. As occurs in animals, durations of no cturnal periods of active melatonin secretion (11.9 +/- 1.6 vs. 10.3 /- 1.3 h, df = 14, t = 4.583, P < 0.0005, paired t test), high prolact in secretion (12.9 +/- 2.1 vs. 9.9 +/- 2.2 h, df = 11, t = 2.917, P < 0.01), and sleep (10.6 +/- 0.8 vs. 7.6 +/- 0.4 h, df = 14, t = 17.122, P < 0.0005) were longer after exposure to long nights than after shor t ones. Durations of nocturnal periods of low rectal temperature (11.6 +/- 2.3 vs. 9.5 +/- 1.6 h, df = 12, t = 3.912, P < 0.001) and rising cortisol secretion (10.8 +/- 1.6 vs. 9.3 +/- 1.9 h, df = 14, t = 3.130 , P < 0.005) were also longer. Some of these differences persisted dur ing 24-h periods of enforced wakefulness in constant dim light, indica ting that prior exposure to the two regimes induced abiding changes in the timing of internal processes, such as circadian pacemaker oscilla tions, that control the durations of nocturnal and diurnal periods of the rhythms.