RHYTHMS THROUGH LIFERHYTHMS THROUGH LIFE
Development of circadian rhythms
Not all organisms have a circadian clock that is active from birth. The question then arises – when does the clock start to tick?
In newly hatched chicks, circadian rhythms such as N-acetyltransferase can be detected from their first exposure to a light-dark cycle. Indeed cycles can be detected in chicken embryos at 17 days of incubation if the eggs are incubated in a light-dark cycle. Chicks however are quite self-sufficient and can survive away form their mother providing that they are kept warm and have adequate food and water (Binkley 1997).
With less self-sufficient animals, circadian rhythms do not seem to be established at birth. Rat pups for example do not show evidence of an N-acetyltransferease cycle until they are 4 days old. Movement cycle do not appear until at least 10 days of age in rat pups separated from their mothers; however rat pups kept with their mothers show these rhythms earlier possible because they are synchronised by the mother (Binkley 1997).
It is well known that newborn humans are not born with an in built sleep-wake cycle – hence why they wake their parents up in the middle of the night. During the early weeks of life the sleep-wake cycle freeruns, with newborns sleeping for up to 16 hours a day. By about 6 weeks of age, the sleep-wake cycle has a period of about 15 hours, diminishing to 24 with time. (Kleitman and Englemann 1953)
The effect of age on cycles
As an animal approaches old age, the amplitude of many of its rhythms decrease. This includes body temperature (seen in mice and rats, and in humans), oxygen consumption (mice), potassium excretion (humans) and the production of melatonin and hormones such as human growth hormone, testosterone and human luteinising hormone (humans), whilst in humans insulin production increases. There can also be an internal desynchronisation of rhythms (Reiter et al. 1980, Davis 1981)
The reduction in amplitude in cycles during age may partly explain why older humans tend to have more sleep disorders such as less daily sleep, more frequent awakenings in the night, a phase shifts in the sleep-wake cycle resulting in the person going to sleep earlier and waking earlier, and needing longer naps during the day. This may be either a direct disruption in the sleep-wake cycle or a disruption in the production of hormones that regulate sleep.
