The Development of Circadian Rhythms in Human Infants

1.7.5 Factors Affecting Cortisol Secretion
Stress has a direct effect on cortisol production via the hypothalamic pituitary adrenal
axis. It switches off the negative feedback loop and results in an increase of cortisol
production.
Infants with mature cortisol axes are more likely to have longer undisturbed periods of
night time sleep.
Having small frequent feeds as opposed to larger volumes less frequently, when
introduced into the model used by de Weerth did not significantly promote or delay
the maturation of the circadian cortisol rhythm but the timing of solids increases
overall cortisol levels (de Weerth et al 2003). The analysis used to formulate these
conclusions was a complex form of modelling using data from 14 infants; within
which there was limited heterogeneity; which may affect the validity of these
conclusions, with regards there application to the general population.
Identifying factors, which may influence circadian rhythmicity of cortisol secretion,
and rhythmicity for the other parameters measured in this study is important, as there
may have a role in entrainment of the infant clock by different infant care practices
(see discussion 11.5). This may have long term impact on infant physiological
development, illness and disease.
The above described aspects of physiology were measured during night time sleep. A
fuller description of sleep and its ontogeny are described in the next section.
30