The Development of Circadian Rhythms in Human Infants

298.051 nmol.l -1 .mg.ml -1 night values 172.562 nmol.l -1 .mg.ml -1 ; Trend p=0.061;
Wilcoxon signed ranks).
It is interesting that there seems to be a diurnal rhythm in cortisol, which was present
and sustained from early on in the time course of the study. It may be that cortisol is
pivotal to maturation of other physiological systems within the body to a degree that
has not previously been highlighted. High levels of cortisol early on in the life of a
newborn baby may confer stability to the infant‟s physiology, evident by the
achievement of a mature temperature pattern.
Studies have reported cortisol rhythmicity occurring from 4 weeks , 8 weeks (de
Weerth et al., 2003, Santiago et al., 1996, Vermes et al., 1980), 12 weeks of age
(Mantagos et al., 1998, Price et al., 1983), or as late as 6-9 months (Onishi et al.,
1983). This difference between the age of emergence of rhythmicity for cortisol in
this study and in other reported studies, may be due to the fact that there is generally
little agreement or consistency in the manner in which a diurnal rhythm is denoted in
the different studies (de Weerth et al., 2003) .
The illustrative paper by De Weerth‟s group, highlighted the inherent difficulty of
comparing findings from different research papers. They applied three different
methods for examining the emergence of a cortisol circadian rhythm as described by
Price, Santiago, Antonini and Krieger, on the same data set. There was little
consistency or agreement as to whether a rhythm had actually appeared when the
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