Seasonal effects on mood have been observed throughout much of human history. Seasonal changes in animals and plants are largely mediated through the changing photoperiod (i.e., the photophase or duration of daylight). We review that in mammals, daylight specifically regulates SCN (suprachiasmatic nucleus) circadian organization and its control of
melatonin secretion. The timing of
melatonin secretion interacts with gene transcription in the pituitary pars tuberalis to modulate production of TSH (
thyrotropin), hypothalamic T3 (
triiodothyronine), and tuberalin
peptides which modulate pituitary production of regulatory
gonadotropins and other
hormones.
Pituitary hormones largely mediate seasonal physiologic and behavioral variations. As a result of long winter nights or inadequate illumination, we propose that delayed morning offset of nocturnal
melatonin secretion, suppressing pars tuberalis function, could be the main cause for
winter depression and even cause depressions at other times of year. Irregularities of circadian sleep timing and thyroid homeostasis contribute to depression. Bright light and sleep restriction are
antidepressant and conversely, sometimes trigger
mania. We propose that internal desynchronization or bifurcation of SCN circadian rhythms may underlie rapid-cycling manic-
depressive disorders and perhaps most
mania. Much further research will be needed to add substance to these theories.