AJP - Regulatory, Integrative and Comparative Physiology, Vol 265, Issue 4 846-R857, Copyright © 1993 by American Physiological Society
Conservation of photoperiod-responsive mechanisms in humans
T. A. Wehr, D. E. Moul, G. Barbato, H. A. Giesen, J. A. Seidel, C. Barker and C. Bender
Clinical Psychobiology Branch, National Institute of Mental Health, Bethesda, Maryland 20892.
In animals, circadian pacemakers respond to seasonal changes in day length
by making corresponding adjustments in the durations of diurnal and
nocturnal periods of circadian rhythms; these adjustments mediate effects
of photoperiod on breeding and other seasonally recurring phenomena. Little
is known about photoperiod responses of human circadian pacemakers. To
investigate this question, we recorded and compared circadian rhythm
profiles of 15 individuals after chronic exposures to short (8 h) and long
(14 h) nights. As occurs in animals, durations of nocturnal 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 prolactin 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 short 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 during 24-h periods of enforced wakefulness in
constant dim light, indicating that prior exposure to the two regimes
induced abiding changes in the timing of internal processes, such as
circadian pacemaker oscillations, that control the durations of nocturnal
and diurnal periods of the rhythms.
