The pineal gland is a neuroendocrine gland that synthesizes and secretes melatonin ( M-acetyl-5-methoxytryptamine). y The afferent input to the pineal gland is transmitted from the retinal photoreceptors through the SCN and sympathetic nervous system. The circadian rhythm of melatonin is controlled by the SCN but is strongly entrained by light. The two effects of light are, first, to regulate melatonin secretion in accordance with diurnal light-dark cycles and, second, to suppress melatonin if given in brief intense pulses. Melatonin secretion increases during the night, reaching a peak level between 2:00 and 4:00 am, then gradually falls during the latter part of the night, and is present at very low levels during the day. Exogenous melatonin has been used with some success to avoid jet lag and may be useful for treatment of phase-shifted sleep and sleep disturbance due to shift work. Melatonin is available through health food stores and has received strong public attention. However, at this time, there are no proven indications for melatonin.
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