The circadian pacemaker imposes stereotypic patterns of daily variation on the activity of human neuroendocrine systems. In a number of cases, these patterns exhibit waveforms that are characterized by distinct diurnal and nocturnal periods with relatively discrete transitions between them (corresponding to a biological day, a biological dusk, a biological night, and a biological dawn). In humans, for example, diurnal periods of absence of melatonin secretion, low prolactin secretion, and falling levels of cortisol alternate with nocturnal periods of active melatonin secretion, high prolactin secretion and rising levels of cortisol. In response to light, the circadian pacemaker synchronizes the timing of the biological day and night so that their timing and duration are appropriately matched with the timing and duration of the solar day and night. As the pacemaker carries out this function, it is able to adjust the duration of the biological day and night to match seasonal variation in the duration of the solar day and night. Thus, after humans have been chronically exposed to long nights (scotoperiods), the duration of nocturnal periods of active melatonin secretion, high prolactin secretion and rising levels of cortisol is longer than it is after they have been chronically exposed to short nights. Furthermore, the sleep-related peak of growth hormone secretion is half as high after exposure to long nights as it is after exposure to short nights. These responses to seasonal changes in duration of the natural scotoperiod are suppressed in most individuals - especially men - who live in modern urban environments in which they are exposed to artificial light after dark and artificial darkness during the daytime.