AO1

• Endogenous pacemakers are internal body clocks that regulate many of our biological rhythms.
• Exognous zeitgebers are external cues that may affect or entrain our biological rhythms.
• The SCN is a tiny bundle of cells in the hypothalamus and is one of the primary endogenous pacemakers in maintaining the sleep/wake cycle.
• The SCN recieves information about light even when our eyes are closed which allows the biological clock to adjust to changing patterns of light when we are asleep.
• The SCN passes information about light to the pineal gland that produces melatonin which induces sleep and prevents waking.
• DeCoursey et al destroyed the SCN connections in the brain of 30 chipmunks, returned them to their natural habitat and observed them for 80 days. They found that the s/w cycle of the chipmunks had disappeared and a significant number of them were killed by predators. Most likely they were awake when they should have been asleep so they were vulnerable to attack.
• Ralph et al bred 'mutant' hamsters who had a 20 hour sleep wake cycle. They then transplanted SCN cells from the mutant hamsters into a normal hamster foetuses and found that the normal hamsters also had a 20 hour sleep cycle.
• The chipmunk and 'mutant' hamster studies both demonstrate that the SCN establishes and maintains the circadian sleep wake cycle.
• There are external factors in the environment that reset our body clocks through a process called entrainment.
• The two main exogenous zeitgebers are light and social cues.
• Siffre showed that in the absense of external cues, the free running biological clock that controls the s/w cycle continue to 'tick' in a distinct cyclical pattern, thus sleeping and wakefulness would seem to be determined by an interaction of internal and external factors.
• Campbell and Murphy demonstrated the importance of light on the s/w cycle - they woke 15 ppts at various times and shone light in the back of their knees and found that the usual s/w cycle differed by up to 3 hours. This shows light is a powerful exogenous zeitgeber that does not need to rely on the eyes to apply an influence.
• The importance of social cues on the s/w cycle is best demonstrated through infant sleep patterns.
• New borns rarely have the same s/w cycle as parents but by 16 weeks old the s/w cycle has been entrained (set by external factors). This is caused by schedules from adults, such as meal times and bedtimes acting as exogenous zeitgebers for new-born infants.
• Research has also found that exogenous zeitgebers can control jet lag by adapting to local eating and sleeping times, rather than responding to the body's own sleep and hunger demands. Meal times are a very specific form of social cues and show this research shoes how the s/w cycle can be entrained.