The Development of Circadian Rhythms in Human Infants

1.6.4 Ontogeny of Melatonin Secretion
Melatonin receptors have been identified in the foetal SCN. Several studies have
investigated the presence of melatonin in the blood immediately after birth.
Melatonin has been detected in newborns but there has been no recognisable day night
rhythm in the first few days and weeks; as reported by Kivela, Munoz-Hoyes and
Thomas (Kivela et al., 1990, Munoz-Hoyos et al., 1993, Thomas et al., 1998).
Interpretation of the results from these studies is limited by the manner by which they
were conducted. Participant numbers in one of these studies was small. Kivela et al
included only 19 mother-infant pairs and used a twice a day sampling schedule. The
spread of sampling was very wide (8:00 to 20:00 for day time urine sample and 20:00
to 8:00 for night time sample). This may account for the finding that a diurnal rhythm
was not detected in the first few weeks. Using a minimum number of samples per day
in attempts to minimise disruption and maximise recruitment and retention study
design may compromise study design and make results less interpretable. The study
by Munoz-Hoyos el al demonstrates this. Although the number of participants was
larger (n=119) single samples was taken per infant (for the reasons stated above),
which would make comment on diurnal rhythm within any infant impossible and
meant that different infants were being compared at different times.
The actual age of the emergence of the first age-related day night difference in
melatonin production has been described as occurring at around 3 months of age
(Kennaway, 2000).
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