Mammalian pineal melatonin: a clock for all seasons.

The central role of the pineal gland and its hormone melatonin (MEL) in mammalian photoperiodic responses is discussed in terms of: 1) evidence for the involvement of MEL in photoperiodism, 2) which feature of the MEL secretion profile might be most important for regulating photoperiodic responses, 3) evidence for the modulation of responses to changes in daylength based on previous photoperiod exposure (i.e., photoperiodic history) and 4) how the MEL signal might be processed at its target sites to elicit physiological responses.