from
The Collaborative International Dictionary of English v.0.48
melatonin \mel`a*tonin\ 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