The Development of Circadian Rhythms in Human Infants

5.1.2 Emergence of diurnal rhythm for melatonin secretion
A diurnal pattern of melatonin secretion was characterised by the age when greater
than fifty percent of the total melatonin was secreted at night time (Sivan & Laudon,
2001).
Apart from an initial peak at 5 weeks of age, the percentage of total melatonin
produced at night remains below fifty percent mark for weeks 6 to 8. More melatonin
is secreted at night compared to day from 9 weeks onwards. The persistence of the
night time dominance between 9 and 17 weeks of age is summarised in table 5.1.
Units melatonin measured in ng. mg. ml -1
Age Mean N Min. (%) Max. Std. Deviation Std. Error of
(weeks) (%)
(%) (%)
Mean (%)
5 76.85 2 69.71 84.00 10.105 7.145
6 45.51 6 4.42 70.15 25.629 10.463
7 41.05 16 1.40 82.02 20.298 5.075
8 44.06 20 .68 91.30 29.545 6.606
9 55.30 17 11.40 88.25 22.628 5.488
10 52.48 25 2.59 98.19 26.278 5.256
11 51.79 17 2.99 98.19 23.918 5.801
12 52.06 20 5.42 94.81 30.283 6.771
13 51.27 12 12.31 90.70 26.664 7.697
14 57.80 8 21.06 98.20 26.278 9.291
15 59.73 5 24.96 90.84 26.062 11.655
16 59.15 4 20.00 91.45 36.415 18.208
17 55.48 4 22.61 81.82 29.291 14.645
Total 51.11 156 .68 98.20 26.025 2.0834
Table 5.1 – Distribution of night time (unlogged values) melatonin secretion as a percentage of
total melatonin (night plus day melatonin) for 34 infants for each week of age studied
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