Endogenous Pacemakers and Exogenous Zeitgebers

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

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: Located in the hypothalamus in each hemisphere of the brain and is one of the primary endogenous pacemakers. 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 receives information about light directly from this structure and continues even when our eyes are closed, enabling the biological clock to adjust to changing patterns of daylight whilst we are sleep.

Animal Studies and The SCN: 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. By the end of the study a significant proportion of them had been killed by predators.

Ralph et al (1990): Bred ‘mutant’ hamsters with a 20-hour sleep/wake cycle. When the foetal tissue of the mutant hamsters was transplanted into the brains of normal hamsters, the cycles of the second group defaulted to 20 hours.

The Pineal Gland and Melatonin: Pineal gland is another endogenous mechanism guiding the sleep/wake cycle. During the night, the pineal gland increases the production of melatonin. Melatonin has been suggested as a causal factor in seasonal affective disorder.

Beyond the Master Clock: Research has revealed that there are numerous circadian rhythms in may organs and cells in the body. These peripheral oscillators are found in the organs including the lungs, pancreas and skin. They are influenced by the actions of the SCN, but also act independently. Damiola et al (2000) demonstrated how changing feeding patterns in mice could alter the circadian rhythm cells in the liver by up to 12 hours, whilst leaving the SCN rhythm unaffected.

Interactionist System: Total isolation studies are extremely rare and even then they use artificial light which can still reset the biological clock. In everyday life, pacemakers and zeitgebers interact, and it makes little sense to separate the two for the purpose of research.

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

Light: Key zeitgeber in humans which can reset the SCN and plays a role in the maintenance of the sleep/wake cycle. Light also have an indirect influence on key processes in the body that control such functions 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 by shining a light on the back of 15 participants’ knees. The researchers managed to produce a deviation in the participants’ usual sleep/wake cycle of up to three hours in some cases.

Social Cues: 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 is an effective way for entraining circadian rhythms.

Environmental Observations: The experience of people who live in places where there is very little darkness in summer and very little light in winter have similar sleep patterns all-year round, despite spending 6 months in total darkness.

Case-Study Evidence: 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 his exposure to social cues.