The Development of Circadian Rhythms in Human Infants

Cortisol samples are reported as weekly values. If collected, a second night-day pair
for the same week was excluded from the analysis, to avoid repeated measures
analysis. Unpaired data were discarded.
The night and day secretion of cortisol did not follow a normal distribution.
Therefore values were logged to base 10 to obtain normal distribution.
For ease of illustration, error bars have been omitted from bar charts.
6.1.1 Changes to cortisol secretion relative to postnatal age
There was no overall increase or decrease in night time cortisol secretion with
postnatal age. Random effects modelling (logged values) demonstrated a non
significant 1% increase per week (p=0.702; 95% CI: 4% decrease to 6% increase).
The distribution of night cortisol levels (unlogged) is shown in Figure 6.1.
Figure 6.2 illustrates how the production of morning cortisol varies with postnatal age.
There is no overall increase or decrease in day time cortisol values between 5 and 18
weeks of age. Random effects modelling shows the average weekly change being a
2% decrease per week (95% CI: 7% decrease to 3% increase; p=0.429), using
normalised data.
132