The Development of Circadian Rhythms in Human Infants

presented in table 6.1 do not have a constant number of measurements at each week of
age and the same infant may be represented within the same developmental age
bracket (figures 6.4 and 6.5) more than once. However, again, the random effects
modelling supports the visual impression from the graphs that there is no sustained
rise or fall in cortisol levels with age over the period measured.
The levels of cortisol secretion remained steady throughout the study period. Random
effects modelling showed no overall net increase or decrease in cortisol with age for
day or night time values (p>0.05). This contrasts with the data for melatonin which
showed evidence of increase with age for night time secretion. It may be interpreted
that cortisol levels rose over a period prior to when monitoring commenced and the
initial rise would have been demonstrated if sampling had been commenced earlier.
Alternatively the measurements of cortisol obtained may be a baseline and if further
samples were taken at older age ranges, there may have been a greater change in
cortisol at a later age that was not demonstrated in this study. A long term follow up
study may be needed to confirm this.
For the comparison of median values of cortisol secretion day and night, there was a
recognisable diurnal pattern of cortisol secretion (day cortisol greater than night
cortisol levels) in the weeks before temperature maturation occurred. Day cortisol
peaked at 370 nmol.l -1 .mg.ml -1 whilst night values were significantly lower at 180
nmol.l -1 .mg.ml -1 (p=0.0001; Wilcoxon signed ranks). Once temperature maturation
had occurred day time cortisol continued to exceed night values (median day values
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