The Development of Circadian Rhythms in Human Infants

Further peripheral gene graphical representation
Following the initial visual inspection, the peripheral circadian gene data were
analysed in two further stages:
i) Fitting a diurnal pattern to the observed data, separately for each week at which
each infant was sampled, and calculating the amplitude as a measure of the
strength of the diurnal relationship.
ii) Modelling the amplitude of the diurnal pattern of peripheral gene expression
against the infant‟s age (in weeks), in order to assess whether there is a „natural‟
change-point that might be interpreted as the age of „gene expression‟ maturity
and also, more generally, to examine changes in amplitude with age.
More detailed data analysis was done by fitting the data points for each infant to a
sinusoidal curve. The diurnal pattern chosen to represent gene expression was the
sine-wave (or cosine wave) with a pre defined period of 24 hours (Bjarnason et al.,
2001). This is a routine manner used for reporting clock gene expression.
The diurnal sine-wave pattern that was fitted - separately for each infant - contained
three parameters, to allow for i) the mean value of expression, ii) the amplitude of the
diurnal variation, and iii) the phase of the diurnal variation, corresponding to the time
of day at which the sinusoidal pattern estimated the maximum level of expression to
occur. The estimated amplitude for any infant at any given week was taken as the
primary measure of „goodness of fit‟ of the data to a sinusoidal pattern as it represents
estimated strength of the diurnal relationship in natural units of gene expression. The
secondary measure of goodness of fit used in this study is the „R squared‟ statistic
conventionally used with linear regression, which may be interpreted as the
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