Biological Rhythms: Circadian Rhythms

• Biological Rhythms are biological fluctuations in an organism that corresponds to environmental change.
• These include changes in body temperature, attention and the sleep-wake cycle.
• Biological rhythms can be endogenous (controlled by internal clocks e.g. the suprachiasmatic gyrus) or exogenous (controlled by external, environmental factors e.g. exposure to sunlight). The three types of biological rhythms are circadian, infradian and ultradian.
• Exogenous zeitgebers = External changes in the environment which affect our biological rhythms.
• Circadian Rhythms = A type of biological rhythm which completes one full cycle every 24 hours e.g. the sleep-wake cycle.
• The main example of an exogenous zeitgeber would be light. Changes in light exposure can trigger desynchronisation of a ‘pre-set’ sleep-wake cycle.
• Siffre (1962), who went into a cave on July 16th 1962, completely devoid of natural light. He finished his experiment on September 14th, believing it to be August 20th.
• This demonstrates that prolonged exposure to a strong exogenous zeitgeber such as light, the sleep-wake cycle becomes disrupted and there is a disconnection between psychological time and the clock.
• His sleep-wake cycle did not conform to a cyclical 24 hour-period, but was around 24 hours and 30 minutes, with Siffre himself determining when to sleep and when to eat. This describes a ‘free-running’ circadian rhythm i.e. one which is not affected by exogenous zeitgebers.
• a weakness of this is that Circadian rhythms do not always have to be 24 hour periods. For example, Professor of sleep medicine at Oxford University, delayed the starting time of High School to 10 AM, instead of the usual 8AM. The researchers noted that disadvantaged students, the rates of scoring 5 A*-C grades increased from 12% to 42%”. Therefore, this is a real-life example of how the circadian rhythms of teenagers specifically are not always in line with that of adults, and so an appreciate of this can improve education.
• Siffre’s research was conducted at a time where researchers believed that artificial lighting had no effect on biological rhythms. The use of artificial light meant that over 2 months, Siffre could have created his own circadian rhythm through signalling sleeping and waking times by using the light, meaning that the conclusions made about his ‘free-running’ circadian rhythm may not be entirely accurate.
• A strength of Siffre's research is that he remained isolated throughout the entirety of the experiment, therefore any changes observed were solely due to the lack of light rather than other external factors influencing him
• Another limitation of Siffre's research is that he only spent two months in complete darkness, whereas some people spend years living in places like Antarctica without sunlight. It is unclear whether someone would experience similar effects if they lived in isolation for longer periods of time.