The Development of Circadian Rhythms in Human Infants

6.1.2 Emergence of diurnal rhythm for cortisol secretion
As for melatonin, the first appearance of a diurnal pattern of cortisol secretion was
characterised by the age when greater than fifty percent of the total cortisol was
secreted during the morning period (Sivan & Laudon, 2001).
Apart from a value of 43% at 14 weeks of age, consistently more cortisol was
produced during day hours from 6 weeks of age till 17 weeks of age (3 weeks earlier
than data for night time melatonin production). These results are summarised in table
6.1.
Units cortisol measured in nmol.l -1 .mg.ml -1
Age Mean N Min. (%) Max. Std. Deviation Std. Error of
(weeks) (%)
(%) (%)
Mean (%)
5 45.73 1 46 46
6 59.28 6 11 89 28.881 11.791
7 65.25 19 24 89 17.539 4.024
8 61.11 21 5 94 22.860 4.989
9 57.51 17 7 86 25.181 6.107
10 62.69 21 20 85 16.659 3.635
11 50.94 14 1 85 26.865 7.180
12 63.41 19 38 95 14.568 3.342
13 62.91 18 10 84 21.519 5.072
14 43.49 6 12 79 26.961 11.007
15 65.23 5 51 86 13.489 6.032
16 61.70 4 21 80 27.284 13.642
17 64.09 2 58 70 8.878 6.278
18 62.73 1 63 63
Total 60.35 154 1 95 21.073 1.698
Table 6.1 - Distribution of day time (unlogged values) cortisol secretion as a percentage of total
cortisol (day plus night cortisol) for 34 infants for each week of age studied
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