The Development of Circadian Rhythms in Human Infants

1.6 Melatonin
(Reviewed in Cardinal 1998)
Melatonin is a primary chemical messenger of the internal „clock‟ which controls all
physiological systems. It plays a key and central role in transmitting and translating
messages regarding the external environment to the central nervous system which in
turn allows circadian rhythms and peripheral oscillators to be synchronised. The
infants‟ night time sleep was selected as the period for study in this project as it is
during night time sleep that the evolution of circadian rhythms in infancy can be
mapped. It is also during night time sleep that the predominance of cot deaths occurs
(Blair et al., 2006), which is why it was originally chosen by this research group as its
primary focus.
1.6.1 Production and Regulation of Melatonin Secretion
Melatonin, a methoxyindole, is mainly made and secreted by the pineal gland during
darkness. It is thought to be the primary messenger of the body which conveys
messages regarding light and dark to cells in the body, specifically retinal cells in the
eye. It helps stabilise circadian day and night rhythms and may determine patterns of
physiological development. It is an important signal of the central biological clock.
Melatonin is made from tryptophan and is found in circulating blood. Tryptophan is
converted to 5-hydroxy tryptophan by hydroxylase enzyme. The product made as a
result of this reaction is converted to serotonin. As a result of the action of two
enzymes, namely sNAT and HIOMT, serotonin is converted to melatonin (Reiter,
1991, Vijayalaxmi Jr et al., 2002). Melatonin is not stored in the body. Therefore if
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