and wakefulness, and is synchronized with the light-dark environmental rhythm. Refreshing sleep is in synchrony with the person’s circadian rhythm. When the circadian clock and the environmental cycle are misaligned, as in jet lag or shift work, sleep disorders may result.
Melatonin, produced by the pineal gland during the dark hours, is equally important in maintaining a normal sleep cycle. Often called the hormone of darkness, mela- tonin has a role in sleep induction because it is released in response to changes in light and inhibits the major neurotransmitters involved in arousal: histamine, norepinephrine, dopamine, and serotonin.
The sleep-wake cycle also is synchronized with other biologic rhythms: body temperature fluctuations and the re- lease of cortisol and growth hormone. Temperature changes correlate with the onset of sleep, the periods of deepest sleep, and the tendency to awaken. The release of cortisol and growth hormone also vary in a circadian pattern according to sleep cycle phases. Because biologic rhythms are synchronized, a disturbance in the normal sleep cycle can affect the neuroendocrine and immune systems.