When seasonal functions such as reproduction, pelage (coat growth and color), appetite, bodyweight are primarily timed by daylength, species are referred to as photoperiodic . Photoperiod is often critical for the timing of pubertal development (38)(39). It is now clear that in photoperiodic mammals and marsupials, an intact innervated pineal gland is essential for the perception of photoperiod change (39, 40).

It is possible to administer melatonin by daily infusion or feeding so as to generate at will circulating profiles, with a duration characteristic of particular photoperiods, in the intact or pinealectomized animal (40, 41). In this way it has become clear that a particular melatonin duration is the necessary and sufficient condition for the induction of a given seasonal response and is equipotent with a particular photoperiod. Long-duration melatonin is equivalent to short days and short-duration melatonin is equivalent to long days (Fig. 2, see above). The interpretation of the signal, as with daylength, depends on the physiology (for example, long- or short-day breeder) of the species in question. In sheep, melatonin can time the whole seasonal cycle, at least of reproduction, acting as a seasonal zeitgeber for a presumed endogenous annual rhythm (42).

Reproduction in domestic ruminants and the winter coat of animals such as mink, arctic foxes, and cashmere goats has commercial significance and can be manipulated by photoperiod and melatonin administration. Implanted melatonin induces short-day effects,and a number of commercial preparations of melatonin have been developed to this end.

Photoperiod via melatonin secretion determines the timing of puberty in some species, provided that a sufficient degree of physical maturity has been reached (38). Interestingly, photoperiod perception by the fetus is present before birth in rodents and ungulates and ensures a rate of development appropriate to environmental conditions (43-45). Melatonin injections to the mother can dictate the timing of postnatal reproductive development. In rats injections of melatonin during the late light phase, during a small window in the late dark phase or even using continuous release implants, specifically during the period of pubertal development, delay reproductive maturity in both males and females (46). Full sexual maturity is eventually achieved; thus the system is not permanently compromised. Moreover in vitro melatonin inhibits gonadotropin-releasing hormone (GnRH)-induced luteinizing hormone (LH) release by cultured rat pituitary glands from prepubertal animals (47). These observations constitute the main evidence for a possible causal role of melatonin in the pubertal development of humans.

Melatonin is produced rhythmically by both the pineal and the retina in many lower vertebrates and probably serves as the common humoral signal for circadian organization (19). In mammals its role appears to be modulatory with regard to circadian organisation. Pinealectomy of rodents in constant light leads to disruption of the circadian system (48). In rats pinealectomy increases the rate of re-entrainment to forced phase shifts of the light-dark cycle (49). Interestingly, in humans, pharmacological suppression of melatonin by atenolol enhances the magnitude of light-induced phase shifts (50) and melatonin and light can act in concert to effect a phase shift (51). Thus, a possible conclusion might be that the presence of melatonin determines the rate of adaptation to phase shift. A specific melatonin antagonist is awaited to resolve these questions. Melatonin is also implicated in circadian thermoregulation (see (52) for a review). Many such effects may involve the thyroid gland.