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  • Endogenous Pacemakers and Exogenous Zeitgebers
    • Endogenous Pacemakers
      • A01
        • Endogenous pacemakers are internal body clocks, the main EP in mammals is the SCN located in the hypothalamus
        • Light enters the eyes, even when they are closed, information is sent via these neural pathways to the SCN.
          • Information is then sent via another neural pathway to the pineal gland which is deep in the brain.
            • Low light information from the SCN triggers the pineal gland to release melatonin so levels in melatonin rise.
        • When blood levels of melatonin reach critical level, we sleep. If the endogenous clock is running slow, morning light shifts the clock ahead, putting the biological rhythm in step with the outside world.
        • Albus
          • Found that the SCN is a pair of structures, one in each hemisphere of the brain and each is divided into ventral and dorsal SCN. The ventral SCN is quickly rest but external cues but the dorsal is much less affected by light therefore resistant to reset.
      • A02
        • Morgan
          • Took the SCN from mutant hamsters with 20 hour circadian rhythms and put it in normal hamsters. He found the normal hamsters then displayed the mutant rhythm. This suggests that the SCN is the main EP and supports the role of EP in controlling biological rhythms.
        • Shifre
          • Lived in a cave for a month to remote the effect of exogenous zeitgebers. His biological clock maintained a constant rhythm but it was 25 hours instead of 24 which suggests the role of EPs are important.
            • However, Siffre was still exposed to artificial light and Bovlin found that even normal room lighting can have and effect on biological rhythms. Therefore, Siffre's research may lack validity, limiting the support it offers for the role of EPs.
        • Schochat
          • Schochat's sleep gate' study found a close relationship between sleep propensity and melatonin. 6 Ps spent 29 hrs in a sleep lab, where for 7 minutes every 20 they had to try to sleep. The highest sleep porpensity was found to occur at late evening and the highest level preceded this by 100-120 minutes, thus supports the role of melatonin in sleep propensity.
            • This study took place in a well controlled lab so has high control / high internal validity. However, it was only correlational so it can only show a relationship, not cause and effect.
    • Exogenous Zeitgebers
      • A01
        • Exogenous zeitgebers are external cues such as light and social cues, light is the more dominant cue in humans. Light can rest the bodies main pacemaker (SCN). It can also be reset by other oscillators because the protein CRY which is a part of the protein clock and is light sensitive.
        • Social cues is another EZ, biologists thought this was the main zeitgeber. Daily rhythms appear to be entrained by social conventions, not internal biology. We now know light is the main zeitgeber but also all parts of the body produce their own rhythms and are not primarily reset by light.
        • Zeitgebers for cells in the liver and heart are likely to be meal times as these cells are reset by eating.
      • A02
        • Campbell + Murphy
          • found that shining a light on the back of someone's knee shifts their circadian rhythms.
        • Miles
          • Did a study of a blind man who had been blind from birth and had a circadian rhythms of 24-29 hours, this supports the claim that light is the main EZ.
            • However, it was only a case study so has a very small sample which makes it hard to generalise.
          • Another study on a blind man showed that they have the same sleep / wake cycle as seeing people therefore as the man was born blind he had brain damage so it makes it hard to generalise.
        • Luce + Segal
          • Did a study on the Arctic circle and found that people who live in the Arctic circle sleep for 7 hours a night, despite that during summer the sun never sets. This shows that light is not the only important zeitgeber, and the biological clock is not the only influenced by light.


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