Endogenous pacemakers and exogenous zeitgebers

Sleep/wake cycle definition

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.

Endogenous pacemakers definition

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

Endogenous pacemakers - The Suprachiasmatic Nucleus (SCN)

The SCN is a tiny bundle of nerve cells located in the hypothalamus in each hemisphere of the brain. - 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 an area called 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. It receives information about light directly from this structure. This continues even when our eyes are closed, enabling the biological clock to adjust to changing patterns of daylight whilst we are asleep.

Endogenous pacemakers - Animal Studies and The SCN (Patricia DeCoursey et al, 2000)

The influence of the SCN has been demonstrated in animal studies. - Patricia DeCoursey et al, 2000, destroyed the SCN connections 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 study 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).

Endogenous pacemakers - Animal Studies and The SCN (Martin Ralph et al, 1990)

In another study, Martin Ralph et al, 1990, bred 'mutant' hamsters with a 20-hour sleep/wake cycle. - When SNC cells from the foetal tissue of mutant hamsters were transplanted into the brains of normal hamsters, the cycles of the second group defaulted to 20 hours.

Endogenous pacemakers - The Pineal Gland and Melatonin

The SCN passes the information on day length and light that it receives to the pineal gland. - This is another endogenous mechanism guiding the sleep/wake cycle. - During the night, the pineal gland increases production of melatonin - a chemical that induces sleep and is inhibited during periods of wakefulness. - Melatonin has also been suggested as a causal factor in SAD.

Endogenous pacemakers - Evaluation: weakness

Another limitation is that endogenous pacemakers cannot be studied in isolation. Total isolation studies, such as Michael Siffre's cave study are extremely rare. Also Siffre made use of artificial light which could have reset his biological clock every time he turned his lamp on. In everyday life, pacemakers and zeitgebers interact, and it may make little sense to separate the two for the purpose of research. This suggests the more researchers attempt to isolate

Endogenous pacemakers - Evaluation: Ethics

Animal studies of 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 of, say, chipmunks and hamsters means that generalisations can be made to the human brain, as the mammalian brain has similar structures. However, a more disturbing issue is the ethics involved in such research. The animals in the DeCoursey et al. study were exposed to risk when returned to their natural habitat, and most died as a result.

Exogenous Zeitgebers definition

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

Exogenous zeitgebers

Exogenous zeitgebers are external factors in the environment that reset our biological clocks through a process known as entrainment. - Seen that, in the absence of external cues, the free-running biological clock that controls the sleep/wake cycle continues to ‘tick’ in a distinct cyclical pattern (as in the Siffre study). - → This free-running cycle is then ‘brought into line’ (i.e. entrained) by environmental cues, so there is an interaction of internal and external factors.

Exogenous zeitgebers - Light

Light is a key zeitgeber in humans. It can reset the body's main endogenous pacemaker, the SCN, and thus plays a role in the maintenance of the sleep/wake cycle. - Light also has an indirect influence on key processes in the body that control such functions as hormone secretion and blood circulation.

Exogenous zeitgebers - Light (Scott Campbell and Patricia Murphy, 1998)

Scott Campbell and Patricia 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 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.

Exogenous zeitgebers - Social Cues

Newborn babies' initial sleep/wake cycle is pretty much random. - At about 6 weeks of age, the circadian rhythms begin and, by about 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.

Exogenous zeitgebers - Evaluation: weakness

One limitation is that exogenous zeitgebers do not have the same effect in all environments. The experience of people who live in places where there is very little darkness in summer and very little light in winter tell a different story from the usual narrative. For instance, the Inuits of the Arctic Circle are said to have similar sleep patterns all-year round, despite spending around 6 months in almost total darkness. This suggests the sleep/wake cycle is primarily controlled by endogenous pacemakers that can override environmental changes in light.

Exogenous zeitgebers - Evaluation: Case Study Evidence

Another limitation is evidence that challenges the role of exogenous zeitgebers. Laughton Miles et al, 1977, recount the study of a young man, blind from birth, 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. This suggests that social cues alone are not effective in resetting the biological rhythm.

Exogenous zeitgebers - Evaluation: strength

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 broken sleep at night (Duffy et al, 2015). However, studies have suggested that exogenous factors may be responsible for the changes in sleep patterns amongst older people. Bernadette Hood et al, 2004, found that management of insomnia was improved if elderly people were generally more active and had more exposure to natural light.