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

WordNet (r) 3.0 (2006):

melatonin
    n 1: hormone secreted by the pineal gland