Exogenous zeitgebers

• Exogenous zeitgebers= external factors in the environment that reset our biological clocks through a process known as entrainment.
• Light is a key zeitgeber in humans. It can reset the body’s main endogenous pacemakers, 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.
• Campbell and Murphy demonstrated that light may be detected by skin receptor sites on the body even when the same information is not received by the eyes.
• Campbell and Murphy studied 15 participants who 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 participant’s usual sleep/wake cycle of up to 3 hours in some cases.
• This suggests light is a powerful exogenous zeitgeber that need not necessarily rely on the eyes to exert its influence on the brain.
• Newborn babies‘ initial sleep/wake cycle is pretty much random.
• At about 6 weeks of age, the circadian rhythms begin and at about 16 weeks, babies’ rhythms have been entrained by the schedules imposed by parents, including mealtimes and bedtimes.
• Research on jet lag suggests that adapting to local times for eating and sleeping (rather than responding to own feelings or fatigue/hunger) is an effective way of entraining circadian rhythms and beating jet lag.
• However, 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 the summer and very little light in the winter tell a different story from the usual narrative.
• People who live in the Arctic Circle 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.
• Miles et al. 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.’c such as regular mealtimes, his sleep/wake cycle could not be adjusted. This suggests social cues alone are not effective in resetting the biological rhythm.