Endogenous Pacemakers & Exogenous Zeitgebers

• Define 'endogenous pacemakers'

  Internal body clocks that regulate many of our biological rhythms, such as the influence of the suprachiasmatic nucleus (SCN) on our sleep/wake cycle.
• Define 'exogenous zeitgebers'

  External factors that affect or entrain our biological rhythm, such as the influence of light on the sleep/wake cycle.
• Define 'sleep/wake cycle'

  A daily cycle of biological activity based on a 24-hour period (circadian rhythm) that is influenced by regular variations in the environment such as the alternation of night and day.
• The suprachiasmatic nucleus (SCN)

  - The SCN is a tiny bundle of nerve cells located in the hypothalamus in each hemisphere.

  - It is one of the primary endogenous pacemakers in mammalian species and is influential in maintaining circadian rhythms such as the sleep/wake cycle.

  - Nerve fibres connected to the eye cross in the optic chiasm on their way to the left and right visual area of the cerebral cortex.

  - The SCN lies just above the optic chiasm and receives information about light directly from the optic chiasm. This continues even when our eyes are closed, enabling the biological clock to adjust to changing patterns of daylight when we sleep.
• Animal studies and the SCN

  - DeCoursey et al. (2000) - destroyed the SCN connection in the brains of 30 chipmunks who were then returned to their natural habitat and observed for 80 days. The sleep/wake cycle of the chipmunks disappeared and by the end of the 80 day period, a significant proportion of them had been killed by predators (presumably because they were awake, active and vulnerable to attack when they should have been asleep).

  - Ralph et al. (1990) - golden hamsters with the Tau mutation have a circadian rhythm of 20 hours compared to 24 hours for 'normal' golden hamsters. When the SCN from normal hamsters was transplanted in the brains of the Tau mutant hamsters, these hamsters gained a sleep/wake cycle of 24 hours. Normal hamsters gained a 20 hour cycle when given the SCN from the mutant hamsters.

  - These 2 studies suggest that the SCN is the main endogenous pacemaker in the sleep/wake cycle.
• The pineal gland and melatonin


  - The SCN passes information on day length and light levels to the pineal gland. This is another endogenous mechanism that guides the sleep/wake cycle.

  - During the night, the pineal gland increases melatonin production - a hormone that induces sleep and is inhibited during periods of wakefulness. Melatonin has also been suggested as a causal factor in seasonal affective disorder.
• Light

  - Light is a key exogenous zeitgeber for humans. It is able to reset the SCN, thus playing a role in the maintenance of the sleep/wake cycle. Light also has an indirect influence on key processes in the body that control functions such as hormone secretion and blood circulation.

  - Campbell and Murphy (1998) demonstrated that light may be detected by skin receptor sites on the body even when the same information is not received by the eyes. 15 participants were woken at various times and a light pad was shone on the back of their knees. The researchers managed to produce a deviation in the participants' usual sleep/wake cycle of up to 3 hours in some cases.

  - This suggests that light is a powerful exogenous zeitgeber that need not necessarily rely on the eyes to exert its influence on the brain.
• Social cues

  - Babies are rarely on the same sleep/wake cycle as the rest of the family, in fact, newborns' initial sleep/wake cycles are quite random.

  - At about 6 weeks of age, the circadian rhythms begin and by around 16 weeks, babies' rhythms have been entrained by the schedules imposed by parents, including adult-determined mealtimes and bedtimes.

  - Research on jet lag suggests that adapting to local times for eating and sleeping, rather than responding to one's own feelings of hunger and fatigue, is an effective way of entraining circadian rhythms and beating jet lag when travelling long distances.
• Evaluating endogenous pacemakers: Beyond the master clock


  - A limitation of SCN research is that it may obscure other body clocks.

  - Research has shown that there are a number of circadian rhythms in many organs and cells. These are known as peripheral oscillators and are found in organs such as the lungs, pancreas and skin. Peripheral oscillators are influenced by the actions of the SCN but act independently as well.

  - Damiola et al. (2000) demonstrated how feeding patterns in mice could alter circadian rhythms of cells in the liver by up to 12 hours, whilst leaving the rhythm of the SCN unaltered.

  - This suggests there are other complex influences on the sleep/wake cycle.
• Evaluating endogenous pacemakers: Interactionist system

  - Another limitation is that endogenous pacemakers cannot be studied in isolation.

  - Total isolation tudies such as Siffre (1975) are extremely rare. Moreover, Siffre also used an artificial light which could reset his biological clock every time he turned the lamp on.

  - In everyday life, pacemakers and zeitgebers interact, and it might make little sense to separate the two for research purposes.

  - This suggests that the more researchers attempt to isolate the effects of endogenous pacemakers, the lower the research validity.
• Evaluating endogenous pacemakers: Ethics


  - Animal studies on the sleep/wake cycle are justified because there are very similar mechanisms at work across species.

  - The existence of an SCN and pineal gland in the brains chipmunks and hamsters for example mean that extrapolations can be made to the human brain, since the mammalian brain has similar structures.

  - However, a more disturbing issue is the ethics involved in the research. The animals in Decoursey et al.'s study were very vulnerable and exposed to risky conditions when returned to their natural habitat, leading to a significant proportion dying.
• Evaluating exogenous zeitgebers: Environmental observations

  - A limitation is that exogenous zeitgebers don't have the same effect in all environments.

  - Some areas such as the Arctic Circle experience little daylight year-round, and yet, maintain similar sleep-patterns all year, despite spending around 6 months in total darkness.

  - This suggests that the sleep/wake cycle s primarily controlled by endogenous pacemakers that can override environmental changes in light.
• Evaluating exogenous zeitgebers: Case study evidence

  - Another limitation is that there is evidence challenging the role of exogenous zeitgebers.

  - Miles et al. (1977) conducted a case study of a blind young man who had an abnormal circadian rhythm of 24.9 hours.

  - Despite exposure to social cues such as regular mealtimes , his sleep/wake cycle could not be adjusted, meaning his sleep was out of synchronisation with the rest of society.

  - This suggests that social cues alone are not effective in resetting the biological rhythm.
• Evaluating exogenous zeitgebers: Age-related insomnia

  - Evidence suggests that people have poorer quality sleep as they get older. This may be due to natural changes in the circadian rhythm as we age, which means falling asleep earlier and having broken sleep throughout the night - Duffy et al. (2015).

  - However, studies have suggested that exogenous factors may be more responsible for the changes in the sleep patterns among older people. Hood et al. (2004) found that the management of insomnia was improved if elderly people were generally more active and had more exposure to natural light.