Biological rhythms - the circadian rhythm

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  • Biological Rhythms: The Circadian Rhythm
    • The Sleep-Wake Cycle
      • Governed by external cues called exogenous zeitgebers
      • Michel Siffre spent 6 months in a cave and found that if the biological rhythm was to free-run then the circadian rhythm would settle down to just over 24 hours. However, it could dramatically change to 48 hours.
      • The studies show that circadian rhythm can persist despite isolation from exogenous zeitgebers. This gives evidence to suggest there is an internal body clock aka endogenous pacemaker.
      • Folkard et al conducted a study to see if external cues could override the internal body clock.
        • 12 people lived in a cave for three weeks. They agreed to go to bed when the clock showed 11:45 and to wake up when the clock said 7:45.
          • As the experiment progressed the clock quickened until it was indicating that 24 hours had passed when only 22 hours had.
            • At the beginning the volunteers circadian rhythm matched the clock but as it quickened their rhythm still followed the 24 hour cycle.
              • Suggests that circadian rhythm can only be guided to a limited extent by external cues.
      • There are individual differences of the sleep-wake cycle.
        • Research has found that circadian cycles can vary from person to person. Some cycles are 13 hours whereas others are 65.
          • This shows that circadian rhythms are not the same in everybody
    • Core-body temperature
      • One of the best indicators of the circadian rhythm.
      • It is at its lowest at around 4:30 AM and at its highest at around 6:00 PM.
      • Slight trough just after lunch. In many countries the practice of having a siesta is related to the dip in body temperature.
      • The circadian variation in core body temperature has been linked to cognitive abilities.
        • Folkard et al looked at the learning ability of 12 and 13 year old children who had stories read to them at either 9:00 AM or 3:00PM.
          • After one week, the afternoon group showed superior recall compared to the morning group.
          • This suggest that long term recall is at its best when body temperature is at its highest.
    • Hormones
      • Cortisol is at its lowest at around midnight and peaks at 6:00 AM.
      • Cortisol is related to making us feel more alert when we wake up and can explain why when we wake up at 4:00 AM it is hard to think clearly.
      • Melatonin and Growth Hormone also peak at around midnight and follow a circadian rhythm.
    • Rhythm that lasts about 24 hours


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