Biological Rhythms

The best example of a circadian rhythm is the 24 hour sleep-wake cycle, associated with which are many cyclical changes with active and dormant periods, for example body temperature. Research has studied whether circadian rhythms are natural and triggered internally (endogenous) or whether they rely upon external cues in the environment (exogenous). Research has involved participants being deprived of possible zeitgebers (an external event that partially controls biological rhythms – literally means ‘time-giver’), such as sunrise and sunset and temperature changes during a 24 period.

Siffre (1972) was removed from the normal light-dark cycle, by being kept in a dark cave for 2 months. There were no zeitgebers such as natural light or sounds and he had no idea what time it was. He had food and drink and so on. His behaviour such as when he slept/woke and when he ate his meals was monitored. At first the findings showed there was no clear pattern in his sleep-waking cycle. However, later his sleep-waking cycle settled down to a regular pattern of about 25 hours i.e. longer the normal 24 hour cycle.

This suggests that our internal biological clock must have a 25 to 30 hour cycle and that that our zeitgebers must reset the clock to our normal 24 hour day. This study is supported by and Wever (1979) who discussed studies on participants who spent several weeks or months in an underground bunker without any cues to light or dark. The findings showed that most of them displayed circadian rhythms of about 25 hours. However, this is not a universal finding. Folkard (1996) studied one individual who had a 30 hour cycle.

These findings suggest that there is an endogenous sleep-waking cycle. However, the fact that there is a difference between the endogenous sleep-waking cycle and the normal sleep-waking cycle indicates that external cues such as changes in light and dark also play a role in entraining our biological clock. Research has been carried out into endogenous factors such as the involvement of the hormone melatonin in the sleep-waking cycle.

Schochat et al (1977) conducted a study where male participants spent 29 hours in a sleep laboratory. Throughout the time, they spent 7 minutes in every 20 lying down in a bed in a darkened room trying to sleep. This allowed Shochat to measure the tendency to sleep at different times of day. Schochat measured the levels of melatonin by taking blood samples three times an hour during the 29 hour period.

At the end of his experiment, his results showed a close relationship between circadian rhythms and melatonin secretion. Therefore, suggesting that melatonin plays a major role in the sleep-waking cycle. Thus, this supports the role of endogenous factors in the sleep-waking cycle. However, these studies have been criticised because they ignore individual differences. There are two important types of individual difference.

One is the cycle length as research has found that circadian cycles can vary anywhere from 13 to 65 hours. The other type of individual difference is whether you are a morning or evening person. Duffy et al (2000) found that morning people prefer to rise early and go to bed early (about 6am and 10pm), whereas evening people prefer to wake and go to bed later (10am and 1am). Furthermore, it has been argued that even though the participants were isolated from exogenous cues, they weren’t isolated from artificial light, which is now known to have an effect on our circadian rhythm. For example, Czeisler at al (1999) altered participant’s circadian rhythms down to 22 hours and up to 28 hours using dim lighting. This challenges the belief that circadian rhythms are endogenous.

A circadian rhythm is a biological rhythm that runs on a roughly 24 hour cycle. One example is the sleep/wake cycle. Humans sleep about 8 out of 24 hours. Even under constant light conditions, animals keep a circadian rhythm (e.g. …

Patterns of behavioural and physiological activity in most organisms vary in a cyclical way. A circadian rhythm is a bodily rhythm that lasts about 24 hours. Other bodily rhythms can be infradian (lasting more than 24 hours) or ultradian (lasting …

Body rhythms are periodically repeated patterns which occur within the body and are governed by internal factors known as endogenous pacemakers, such as hormones and external factors known as exogenous ‘zeitgebers’ (time-givers), such as light and dark. The three main …

Generalisabilty is a huge weakness in studies into Circadian rhythms as the research includes case studies or small samples. However research into Circadian rhythms can improve our understanding of medical conditions for example: migraine sufferers perceived that they were most …

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