Circadian Rhythms

Circadian Rhythms, AO1 and AO2.

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AO1 - The Sleep-Wake Cycle 1

Circadian Rhythms

  • Biological rhythms that last 24 hours
  • The best known ones are the sleep-wake cycle and the body temperature cycle

The Sleep-Wake Cycle

  • Exogenous cues = sunlight, clocks
  • These are not the sole reason that we sleep and wake when we do
  • Psychologists have investigated what happens when a person is free of these external cues
  • Their biological rhythm is allowed to be free-running

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AO1 - The Sleep-Wake Cycle 2

Aschoff and Wever

  • Placed participants in an underground WWII bunker, away from environmental and social time cues
  • Most people displayed circadian rhythms of 24-25 hours, though some were as long as 29 hours
  • Circadian rhythms persist despite isolation from natural light, demonstrating the existence of an endogenous 'clock'
  • External cues are important, however, because the clock was not totally accurate - it varied day by day
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AO1 - The Sleep-Wake Cycle 3

Folkard et al

  • Wanted to see if external cues could be used to override the internal clock
  • 12 people lived in a cave for 3 weeks, isolated from natural light and other time cues
  • They agreed to go to bed when the clock said 11:45pm and get up when it said 7:45am
  • Initially the physical clock ran normally, but they gradually quickened it so that it seemed to be timing 24 hours when it was actually only doing 22
  • At first the volunteers' circadian cycles matched the clock but, as it quickened, it continued to follow a 24-hour cycle and not a 22-hour one
  • This suggests that the circadian rhythm can only be guided to a limited extent by external time cues
  • As soon as the experiment ended, it only took a few days for the participants' cycles to follow the normal clock again
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AO1 - Core Body Temperature 1

  • Core body temperature is lowest (36 degrees) at around 4:30am and highest (38 degrees) at around 6:00pm
  • There is a slight trough just after lunch, even if you have not eaten
  • In many countries the practice of having an afternoon siesta is related to this dip in temperature
  • This temperature dip is a bi-daily ultradian rhythm
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AO1 - Core Body Temperature 2


  • Cortisol makes us alert when we wake up
  • It's lowest around midnight but peaks around 6am
  • This can explain why it's hard to think clearly when we wake at 4am
  • Melatonin (which induces sleep) and growth hormone also have a clear circadian cycle, both peaking around midnight
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AO2 - Research Methodology

  • In all studies, participants were isolated from variables like clocks, radios and daylight
  • However, they were not isolated from dim artificial light, as it was not perceived to have an effect
  • Czeisler et al: lowered participants' circadian rhythms down to 22 hours and up to 28 hours just by using dim lighting
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AO2 - Individual Differences

  • Two important types: cycle length and cycle onset
  • Cycle length: cycles can vary in people from 13 to 65 hours
  • Cycle onset: Duffy et al found that morning people prefer to sleep and wake early (10pm and 6am)...
  • ...whereas evening people prefer to sleep and wake later (1am and 10am)
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AO2 - Core Body Temperature and Cognitive Abilitie

  • Variation in core body temperature has been linked to cognitive abilities
  • Folkard et al: read stories to two groups of 12- and 13-year-olds either at 9am or 3pm
  • After 1 week the afternoon group (higher body temperature) retained about 8% more meaningful material than the morning group
  • Gupta: performance on an IQ test was better at 7pm than at 9am or 2pm, which could be significant when planning exam timetables
  • Giesbrecht et al: placed participants in cold water and found cognitive performance worse on some tasks
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AO2 - Core Body Temperature and Cognitive Abilitie

  • Other research has found this link to be spurious
  • Hord and Thompson: tested cognitive performance in a field rather than a lab situation
  • Found no correlation between core body temperature and cognitive performance
  • Wright et al: higher body temperature leads to increased physiological arousal, and this may lead to improved cognitive performance
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IDA - Real-World Applications

  • Chronotherapeutics is the study of how timing affects drug treatments
  • Since the circadian rhythm affects digestion, heart rate, hormone secretion etc., this should be taken into account when taking drugs
  • Medications that act on certain hormones may have no effect if taken when target hormone levels are low
  • Taking aspirin to treat heart attacks is most effective around 11pm, which allows the aspirin to peak in the bloodstream
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IDA - Free Will and Determinism

  • All these studies are typical of the biological approach
  • They propose that human behaviour can be explained in terms of structures in the brain and hormonal activity
  • However, human behaviour is often more complex than that, because people can override biologically-determined behaviours by making choices
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