*melatonin*

n. (Physiology) A hormone secreted by the pineal gland. Chemically it is N-acety-5-methoxytryptamine. Research has indicated that there are daily rhythms in secretion of melatonin, in particular due to the depressing effect on melatonin production by light received by the retina. Conversely it appears that melatonin may influence the circadian rhythms of animals. There is some experimental evidence that administration of melatonin may increase the amount of sleep in people with sleep disorders. However, the evidence is not convincing and the effect is not profound. Nevertheless, the deregulation of melatonin and its availability over-the-counter has led to many people taking melatonin to help sleep, without consulting a physician. [ WordNet 1.5 +PJC ]

A role for melatonin in sleep facilitation has been inferred from its effect on electroencephalogram patterns, but it has not been possible to demonstrate that wakefulness sleep cycles are driven by periodic accumulation, depletion, or regeneration of melatonin. Uwe Ackermann, Essentials of Human Physiology, 1992