Many of the dramatic seasonal changes that occur in animals are triggered by changes in the length of night. Changes in the duration of melatonin secretion, which usually occurs exclusively at night and therefore lasts longer in winter than in summer, often act as a chemical transducer of the effects of seasonal changes in night-length on animals' behavior and physiology. Cells in sites that regulate seasonal behaviors, such as those associated with breeding, appear to possess melatonin receptors and interval timing mechanisms that enable them to detect melatonin and measure the duration of the nightly interval when it is present. These sites then trigger changes that are appropriate for the season that is indexed by the measured length of the interval.
