Endogenous pacemakers and exogenous zeitgebers

The suprachiasmatic nucleus (SCN) is a tiny bundle of cells which is located in the hypothalamus of each hemisphere, just above the optic chiasm. Nerve fibres that are connected to the eye cross in the optic chiasm. The SCN receives information about light, even when our eyes are closed, enabling the biological clock to adjust whilst we are asleep.

The SCN passes the light information it receives to the pineal gland, which produces melatonin. During the night production of melatonin increases. Melatonin is a chemical which induces sleep.

Animal studies were conducted to investigate the effects of the SCN on the sleep/wake cycle. The SCN connections were destroyed in 30 chipmunks, who were then released back into their natural habitats and were observed for 80 days. It was found that their sleep/wake cycles dissapeared, and as a result of this many were killed by predators due to being awake at times they were not supposed to be.

In another study, hamsters with a 20 hour sleep/wake cycle were bred. When the SCN cells of the foetus mutant hamsters were transplanted into the brains of normal hamsters, there sleep wake cycles adjusted to 20 hours.

Exogenous zeitgebers are external factors in the environment which entrain our biological clocks. Examples of this is social cues, and light.

Light plays a role in the sleep/wake cycle as it can reset the SCN. A study demonstrated that light may be detected by skin receptors on the body, even when the info is not received by the eyes. This highlights how light is a powerful exogenous zeitgeber which influences the sleep/wake cycle.

Social cues also entrain the sleep/wake cycle. At 6 weeks, a babies circadian rhythm begins, and by 16 weeks of age it has been entrained by schedules imposed by adults such as pre-determined mealtimes and bedtimes.

A limitation of research into the SCN is that it may obscure other body clocks. Research has found that there are numerous circadian rhythms in many organs and cells in the body, including the lungs, pancreas, and skin. They are influenced by the SCN, however also act independently. A study found that feeding patterns in mice could alter the circadian rhythms of cells in the liver by 12 hours, whilst leaving the SCN unaffected. This suggests other complex influences on the sleep wake cycle

Another limitation is that endogenous pacemakers cannot be studied in isolation. Total isolations studies such as Siffres cave study are rare and diffiuclt to do. However Siffre did use artificial light (a lamp) which could have reset his biological clock. In everday life endogenous pacemakers and exogenous zeitgebers interact so it does not make sense to isolate the two for research. Therefore research may have lower validity.

A limitation is that exogenous zeitgebers do not have the same effect in all environments. People who live in the Artic circle have very little light in the winter and very little darkness in the summer. Despite this, they have very similar sleep patterns all year round, even though they are living in darkness in the winter. This suggests that the sleep/wake cycle is primarily controlled by endogenous pacemakers that can overide the influence of light.

Another limitation is evidence that challenges the role of exogenous zeitgebers. A young man was studied who had been 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 social cues alone are not effective at entraining the sleep wake cycle.