- Biological rhythms are regulated by endogenous pacemakers,which are the body’s internal biological clocks, and exogenous zeitgebers, which are external cues, including light, that help to regulate the internal biological clocks.
Endogenous Pacemakers
- Endogenous pacemakers are internal mechanisms that govern biological rhythms, in particular, the circadian sleep-wake cycle. Although endogenous pacemakers are internal biological clocks, they can be altered and affected by the environment. For example, although the circadian sleep-wave cycle will continue to function without natural cues from light, research suggests that light is required to reset the cycle every 24 hours. (See Siffre and Aschoff & Weber, above)
- The most important endogenous pacemaker is the suprachiasmatic nucleus, which is closely linked to the pineal gland, both of which are influential in maintaining the circadian sleep/wake cycle.
- The suprachiasmatic nucleus (SCN), which lies in the hypothalamus, is the main endogenous pacemaker (or master clock). It controls other biological rhythms, as it links to other areas of the brain responsible for sleep and arousal. The SCN also receives information about light levels (an exogenous zeitgeber) from the optic nerve, which sets the circadian rhythm so that it is in synchronisation with the outside world, e.g. day and night.
- The SNC sends signals to the pineal gland, which leads to an increase in the production of melatonin at night, helping to induce sleep. The SCN and pineal glands work together as endogenous pacemakers; however, their activity is responsive to the external cue of light. Put simply:
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- As outlined above, exogenous zeitgebers influence biological rhythms: these can be described as environmental events that are responsible for resetting the biological clock of an organism. They can include social cuessuch as mealtimes and social activities, but the most important zeitgeber is light, which is responsible for resetting the body clock each day, keeping it on a 24-hour cycle.
- The SNC contains receptors that are sensitive to light and this external cue is used to synchronise the body’s internal organs and glands. Melanopsin, which is a protein in the eye, is sensitive to light and carries the signals to the SCN to set the 24-hour daily body cycle. In addition, social cues, such as mealtimes, can also act as zeitgebers and humans can compensate for the lack of natural light, by using social cues instead. Exogenous Zeitgebers
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- Strength- The importance of SCN has been demonstrated in research
- Ralph et al bred hamsters so that they had circadian rhythms of 20 hours rather than 24. SCN neurons from these abnormal hamsters were transplanted into the brains of normal hamsters, which subsequently displayed the same abnormal circadian rhythm of 20 hours, showing that the transplanted SCN had imposed its pattern onto the hamsters. This research demonstrates the significance of the SCN and how endogenous pacemakers are important for biological circadian rhythms.
- However, this research is flawed because of its use of hamsters. Humans would respond very differently to manipulations of their biological rhythms, not only because we are different biologically, but also because of the vast differences between environmental contexts
- Strength- Research support for the role of exogenous zeitgebers
- When Siffre returned from an underground stay with no clocks or light, he believed the date to be a month earlier than it was. This suggests that his 24-hour sleep-wake cycle was increased by the lack of external cues, making him believe one day was longer than it was. This highlights the impact of external factors on bodily rhythms
- Limitation- influence of exogenous zeitgebers may be overstated
- Miles et al note the case of a man blind from birth with a circadian rhythm of 24.9 hours- his sleep-wake cycle could not adjust to social cues, so he took sedatives at night and stimulants in the morning to align with the 24 hour world
- Similarly, studies of individuals who live in artic regions (where the sun does not set during the summer months) shown normal sleep patterns despite prolonged exposure to sunlight
- Both these examples suggest there are occasions when exogenous zeitgebers may have little bearing on our internal rhythm
- Limitation- Could be considered Biologically Reductionist
- Despite all the research support for the role of endogenous pacemakers and exogenous zeitgebers, the argument could still be considered Biological Reductionist. For example, the behaviourist approach would suggest that bodily rhythms are influenced by other people and social norms, i.e. sleep occurs when it is dark because that is the social norm and it wouldn’t be socially acceptable for a person to conduct their daily routines during the night. The research discussed here could be criticised for being Reductionist as it only considers a singular biological mechanism and fails to consider the other widely divergent viewpoints.