Endogenous pacemakers and exogenous zeitgebers

Biopsychology

Endogenous pacemakers

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

Exogenous Zeitgebers

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

Endogenous pacemakers: The Suprachiasmatic Nucleus

Tiny bundle of nerve cells in the hypothalamus of each hemisphere

maintains circadian rhythms such as the sleep/wake cycle

SNC receives information about light and continues to do so even when our eyes are closed, enables biological clock to adjust to changing patterns of daylight whilst we are asleep.

Animal studies and the SCN (DeCoursey et al)

Destroyed the SCN connections in chipmunk brains then released them back into their natural habitat and observed then for 80 days. They found the sleep/wake cycle disappeared and many had been killed by predators.

Animal studies and the SCN (Ralph et al)

Bred mutant hamsters which had a sleep/wake cycle of 20 hours. When SCN cells were transplanted into normal hamster brains, the normal hamsters sleep/wake cycle became 20 hours.

The Pineal gland and melatonin

SCN passes information on day length and light to the pineal gland

Endogenous mechanism which guides the sleep/wake cycle

in the night the pineal gland increases its production of melatonin (melatonin is a chemical which induces sleep)

Exogenous zeitgebers: light

light can reset the SCN (the body’s main endogenous pacemaker) to plays a role in maintaining the sleep/wake cycle

exogenous zeitgebers: social cues

Research on jet lag suggests that adapting to local times for eating and sleeping (rather than responding to your own hunger and tiredness) is an effective way of entraining the circadian rhythm and beating jet lag.

Evaluation Siffre

A limitation is that endogenous pacemakers cannot be studied in isolation.

Siffre’s cave study is a rare case and we cannot be certain from his study whether his use of artificial light (an exogenous zeitgeber) may have reset his biological clock.

This means his research has decreased validity as he may not have been researching what he intended to. Pacemakers and Zeitgebers interact in everyday life so there is little use for studies which isolate the two.

Evaluation (using animals in research)

Another limitation is the use of animals to study the influence of the SCN.

Findings from animals (in this case hamsters and chipmunks) cannot be generalised to humans as they are different organisms. Also it is unethical to conduct research on animals. In DeCoursey’s case most of the chipmunks got killed by predators which is unethical as the chipmunks were not protected from harm.

This reduces the use of the studies as findings may not be useful when applied to humans because findings cannot be generalised from animals to humans. This also raises ethical concerns because these animals may have died for nothing if findings are not useful for humans.

Evaluation (environmental observations)

One limitation is that exogenous zeitgebers do not have the same affect in all environments.

People living in the Arctic circle have similar sleep/ wake patterns all year round despite spending 6 months in nearly total darkness.

This suggests that the sleep/wake cycle is mainly controlled by endogenous pacemakers that can override environmental changes in light. This reduces validity of theories which claim exogenous zeitgebers (including light) have a more important role in controlling the sleep/wake cycle in comparison to endogenous pacemakers.

case study evidence limitation

Case study evidence to challenge the role of exogenous zeitgebers is that Miles (a man blind from birth) had an abnormal sleep/wake cycle of 24.9 hours. Despite exposure to social cues (e.g. mealtimes) his sleep/wake cycle could not be adjusted. This suggests that social cues alone are not effective in resetting the biological rhythm which decreases the validity of the theory as other things may impact the sleep/wake cycle more than social cues.