The Development of Circadian Rhythms in Human Infants

1.7.3 Circadian pattern of Cortisol Secretion
In adults, cortisol has a robust day/night rhythm of secretion; with a peak at dawn, in
order to get ready for the „stresses‟ of a new day and a trough occurring normally in
the middle of the night. Melatonin and cortisol are „phase locked‟ with the cortisol
trough occurring at the time of melatonin peak.
The pattern of cortisol secretion is also linked to the sleep wake cycle. A change in
night time sleep onset will shift the timing of the peaks and troughs.
1.7.4 Ontogeny of Cortisol Secretion
A small study confirmed work by previous researchers that a robust day-night pattern
of secretion of cortisol is not present at birth (de Weerth et al., 2003). In other work
conducted in term, premature and small for dates infants, cortisol levels were found to
be high initially straight after delivery. This may confer protection to the infant
during the delivery itself or initial high levels may enable the newborn to cope with
the rapid adaptation required ex-utero in the brand new external environment. The
foetal adrenal gland then involutes quickly within the first week with an
accompanying drop in levels (Rokicki et al., 1990, Atkinson, 1995).
The amount of cortisol then produced starts to rise again over the next few weeks in
response to increased ACTH production. This results in a tripling or quadrupling in
blood cortisol levels (Winter, 1985). The infant then begins to develop a circadian
pattern over the first 4 postnatal months of life.
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