The Development of Circadian Rhythms in Human Infants

crucial in physiological development and maturation. However it should be noted that
the study did not include extremely premature infants. It included only nine infants
born between 31 and 34 weeks gestation. The HPA axis of extremely premature
infants would be expected to be significantly immature and not likely to mature at a
similar postnatal age to term infants.
The visual data were based on working definitions of the appearance of a diurnal
pattern of secretion for the secretion of the hormones melatonin and cortisol, sleep
and peripheral gene expression.
Detailed statistical analysis to determine the exact timing of the maturation of each
aspect of physiology studied was not feasible due to the sample size, and the inherent
difficulties comparing different types of outputs. For example it is problematic to
attempt to link hormone secretion directly to gene expression; whilst comparing age
of maturation of cortisol secretion with melatonin secretion by determining timing of
maximal diurnal rhythmicity (as there are both humoral outputs) is more
straightforward. Although there are limitations as to what can be inferred from the
data, there is a possible suggestion that the physiological maturation of the different,
but interrelated systems, occurs over a period of weeks, rather than simultaneously.
The box and whisker plots show there is wide variability within each physiological
system measured for the age at which „maturation‟ took place but does not clearly
give any indication of the precision of the estimate of the mean for each week of
maturation. This study does, however, contribute strongly towards the argument for
larger studies, to more accurately answer the question whether there is actually a
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