Sleep-wake patterns of 20 normal, healthy infants (16 girls and four b
oys; age range: 26-37 weeks) were recorded for a period of 1 week with
a miniature activity monitor. Urine samples were extracted from the i
nfants' 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'' sec
retion patterns (i.e. with an adult-like circadian rhythm) had a signi
ficantly delayed sleep-wake cycle in comparison to those with ''immatu
re'' 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 rat
es 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 th
e night (e.g. r = 0.49; p < 0.05; for lower sleep percent). These find
ings suggest that melatonin plays an important role in the evolution o
f the sleep-wake system.