The Development of Circadian Rhythms in Human Infants

due to small sample size and limits on data collection within the home setting. For
tighter definitions for age of circadian maturity in melatonin secretion, cortisol
secretion, maturity of infant sleep and age circadian gene expression matures occurs,
more detailed, larger home based studies plus research performed under lab
conditions need to be conducted.
Core body temperature physiological maturity is said to be achieved when an infant is
able to drop core body temperature to a minimum of 36.4 C during the first four hours
of night time sleep (Lodemore et al., 1991).
The beginning of an emergence of a diurnal rhythm of melatonin excretion may be
illustrated by demonstration of a day night difference in melatonin secretion for each
infant. This was defined when more than fifty percent of total melatonin was secreted
during the night time period, or when night time secretion exceeded day time levels
(Sivan et al., 2001). The initial appearance of diurnal rhythmicity was defined when
the first peak in night time melatonin occurred.
Cortisol data were analysed in a similar fashion. The timing of a diurnal pattern of
cortisol secretion was defined as the week the morning cortisol secretion was more
than fifty percent of the total cortisol production. This characterised the first cortisol
peak and the first suggestion appearance of diurnal rhythmicity.
The pattern of an age related change in peripheral gene expression was characterised
by examining values of amplitude and R squared. The age when there is a combined
rise in amplitude of gene expression to above 0.05 and goodness of fit to sine wave
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