Developing circadian rhythmicity in infants.

Circadian rhythms are endogenously generated rhythms with a period length of about 24-hrs. Evidence gathered over the past decade indicates that the circadian timing system develops prenatally and the suprachiasmatic nuclei, the site of a circadian clock, are present by mid-gestation in human and non-human primates. Recent evidence also shows that the circadian system of primate infants is responsive to light at very premature stages and that low intensity lighting can regulate the developing clock. After birth, there is progressive maturation of the circadian system outputs, with pronounced rhythms in sleep-wake and hormone secretion generally developing after two months of age. With the continued elucidation of circadian system development and influences on human physiology and illness, it is anticipated that consideration of circadian biology will become an increasingly important component of neonatal care.