Circadian systems in a wide variety of organisms all appear to include thre
e basic components: 1) biological oscillators that maintain a self-sustaine
d circadian periodicity in the absence of environmental time cues; 2) input
pathways that convey environmental information, especially light cues, tha
t can entrain the circadian oscillations to local time; and 3) output pathw
ays that drive overt circadian rhythms, such as the rhythms of locomotor ac
tivity and a variety of endocrine rhythms. In mammals, the circadian system
is employed in the regulation of reproductive physiology and behavior in t
wo very important ways. 1) In some species, there is a strong circadian com
ponent in the timing of ovulation and reproductive behavior, ensuring that
these events will occur at a time when the animal is most likely to encount
er a potential mate. 2) Many mammals exhibit seasonal reproductive rhythms
that are largely under photoperiod regulation; in these species, the circad
ian system and the pineal gland are crucial components of the mechanism tha
t is used to measure day length. The rhythm of pineal melatonin secretion i
s driven by a neural pathway that includes the circadian oscillator(s) in t
he suprachiasmatic nuclei. Melatonin is secreted at night in all mammals, a
nd the duration of each nocturnal episode of melatonin secretion is inverse
ly related to day length. The pineal melatonin rhythm appears to serve as a
n internal signal that represents day length and that is capable of regulat
ing a variety of seasonal variations in physiology and behavior. (C) 1999 E
lsevier Science Inc. All rights reserved.