The pineal hormone melatonin plays a ubiquitous role in biology as a chemical mediator of the effects of season on animal physiology and behavior. Seasonal changes in night length (scotoperiod) induce parallel changes in the duration of melatonin secretion (which occurs exclusively at night), so that it is longer in winter and shorter in summer. These changes in duration of nocturnal melatonin secretion, in turn, trigger seasonal changes in behavior. The retinohypothalamic-pineal (RHP) axis's responses to light are highly conserved in humans. Like other animals, humans secrete melatonin exclusively at night, and they interrupt its secretion when they are exposed to light during the nocturnal period of its secretion. In many individuals, the RHP axis also is capable of detecting changes in the length of the night and making proportional adjustments in the duration of nocturnal melatonin secretion, producing the type of melatonin message that animals use to trigger seasonal changes in their behavior. This has been shown both in naturalistic studies in which melatonin profiles were compared in summer and winter and in experimental studies in which melatonin profiles were compared after chronic exposure to long and short artificial "nights." Individuals who live in modern urban environments differ in the degree to which, or even whether, the intrinsic duration of melatonin secretion (the duration measured in constant dim light) responds to seasonal changes in the length of the solar night. Changes in the intrinsic duration of melatonin secretion that are induced by changes in the scotoperiod are highly correlated with changes in the intrinsic timing of the morning offset of secretion and are only weakly correlated with changes in the intrinsic timing of evening onset of secretion. This finding suggests that differences in the way in which individuals are exposed to, or process, morning light may explain differences in their responsiveness to changes in duration of natural and experimental scotoperiods. Although the human RHP axis clearly is capable of detecting changes in the length of the night and in producing the melatonin message that other animals use to trigger seasonal changes in their behavior, it is not yet known whether or how the human reproductive system or other systems respond to this message.