The body will naturally go through ‘Rhythms’, how frequent these rhythms are will determine what type they are.
Infradian rhythms – have cycles that occur less than once every day – i.e. the menstrual cycle.
Circadian rhythms – have cycles that occur once every 24 hours – i.e. the sleep-waking cycle
Ultradian rhythms – have cycles that occur more than once every 24 hours – i.e. the different stages within sleep.
What controls these rhythms?
The biological rhythms are controlled by factors inside and outside our bodies; these are known as endogenous pacemakers and exogenous zeitgebers.
Endogenous pacemakers – These are the factors inside our body that regulate our biological rhythms. Within the hypothalamus there is the suprachiasmatic nucleus (SCN), this is believe to control the pineal gland which secretes the hormone melatonin (which induces sleep).
Psychologists have found that melatonin levels are lowest during the light hours and highest during dark hours. This suggests the SCN controls the levels of melatonin to make sure we sleep during the night and stay awake in the day.
Menaker et al (1978) lesioned the SCN in hamsters and found that their sleep-wake cycle was disrupted. This supports the view that endogenous pacemakers (at least partly) control our biological rhythms, because without it the hamsters’ rhythms were disrupted.
Exogenous zeitgebers – These are factors outside our body that may trigger a biology rhythm. The most common example of this is light. Michael Siffre spent six months in a cave, without clocks or natural light. His sleep-wake cycle went from 24-hours to between 25 and 30 hours. This suggests that exogenous zeitgebers such as light are needed to keep our rhythms in sync.
The interaction between endogenous and exogenous factors
Usually both the endogenous and exogenous factors will interact to determine a biological rhythm. However, in some cases endogenous factors may completely determine a rhythm. Pengelly and Fisher (1957) found that squirrels will hibernate even when in a laboratory, with completely different conditions to the normal habitat. This shows endogenous factors can completely determine a cycle because the exogenous factors changed but the biological rhythm remained the same.