The Development of Circadian Rhythms in Human Infants

1.3.1 Detection of Rhythm
In order to detect a rhythm a variable must be measured over more than one cycle and
the cycle must be shown to be repeated. Adequate and frequent sampling is
necessary.
1.3.2 Exogenous and Endogenous Rhythm
(Adapted from Minors 1981)
Exogenous rhythms are characterised by the environment and outside factors which
are thought to control them. If the external factor is taken away or a change made to
the environment which is critical, then the rhythm will fail to exist.
Endogenous rhythms have an internal driving factor which „compels‟ it to run and
directs it. It does not depend on the environment and will continue to oscillate if all
external factors are obliterated or extinguished. They have an internal „clock‟ or
„pacemaker.‟
Entrainment is the process by which factors which are not governed by an internal
pacemaker; but which are related to the environment (known as zeitgebers or time
givers) which synchronise or change the timing of a rhythm. An example is darkness
which is a cue giver for the secretion of melatonin within in the body (Minors, 1981).
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