Sleep and melatonin in infants: a preliminary study.

Sleep-wake patterns of 20 normal, healthy infants (16 girls and four boys; age range: 26-37 weeks) were recorded for a period of 1 week with a miniature activity monitor. Urine samples were extracted from the infants' disposable diapers that were collected during a 60-hour period to determine the levels of 6-sulphatoxymelatonin (aMT6s, a melatonin metabolite) using a radioimmunoassay test. Infants with "mature" secretion patterns (i.e. with an adult-like circadian rhythm) had a significantly delayed sleep-wake cycle in comparison to those with "immature" patterns. The onset of their nocturnal-sleep episode was delayed by almost 1 hour (22.1 vs. 21.2 hours; p < 0.05). Higher secretion rates of aMT6s during the evening hours (6:00-10:00 p.m.) were associated with earlier onset of nocturnal sleep (r = 0.51; p < 0.05). A delayed peak of melatonin was associated with more fragmented sleep during the night (e.g. r = 0.49; p < 0.05; for lower sleep percent). These findings suggest that melatonin plays an important role in the evolution of the sleep-wake system.