The Development of Circadian Rhythms in Human Infants

NREM sleep has 4 stages. It occupies the majority of adult sleep time. As an
individual progresses from stage 1 to 4 of sleep, the depth and level of
unconsciousness increases. There are characteristic physiological changes which
occur. Heart rate and core body temperature fall as do systolic blood pressure and
respiratory rate. Parasympathetic nervous tone increases whilst there is a
corresponding fall in sympathetic tone. NREM sleep has characteristic external
physical changes which can be observed. Although there is a slower rate of breathing,
the respiratory pattern in NREM sleep is regular. Eye movements are slow (hence
the name non rapid eye movements). Rather than be flaccid and atonic, body muscle
tone is preserved.
During REM sleep, overall sleep physiology is similar to NREM (in comparison to
wakefulness), but there are some distinct differences. Breathing is more irregular and
eye movements rather than being slow, are rapid. Total body tone is reduced which
acts as a protective mechanism and prevents one from enacting one‟s dreams.
In infants, sleep states have alternative nomenclature. Sleep is divided into Quiet
Sleep (QS or Non REM sleep equivalent) and Active Sleep (AS or REM sleep
equivalent).
All of the above sleep stages, whether being observed in children or adults, have set
criteria which allow the appropriate sleep stage to be assigned. Full detail of the
electrophysiological changes in corresponding sleep stages are found in published
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