Biological rhythms

Biological rhythms

  • Biological rhythms are regular cyclical changes in our biological systems.
  • Rhythms repeat over different lengths of time
  • All biological rhythms are governed by 2 things:
    • 1) the body's internal biological 'clocks' = endogenous pacemakers 2) external changes in the environment = exogenous zeitgebers
  • Biological rhythms vary in length:
    • Circadian rhythms (CR) have cycles that generally occur once every 24 hours - for instance the sleep/wake cycle
    • Infradian rhythms (IR) have cycles that occur less than once a day - for instance the mentrual cycle
    • Ultradian rhythms (UR) have cycles that occur more than once every 24 hours - for example the sleep cycle has several repeating stages of light and deep sleep
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The sleep/wake cycle

  • Circadian rhythms are those rhythms that last for around 24 hours
  • This includes the the sleep/wake cycle
  • When we feel drowsy at night and alert during the day shows the effect of daylight = daylight is an important exogenous zeitgeber on our sleep/wake cycle
  • What if there was no influence of an external stimuli such as light on our biological clock?
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Siffre's Cave Study

  • Siffre spent several extended periods underground to study the effects on his own biological rhythms
  • He was deprived of exposure to natural light and sound
  • After 2 months in the caves, he believed it to be mid-August 
  • His study shows how his free running biological rhythm (BR) settled down to one that was just beyond the usual 24 hours (around 25 hours) 
  • He also did continue to fall asleep and wake up on a regular schedule
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Other Research

  • Aschoff and Wever found similar results to Siffre
  • They convinced a group of participants to spend 4 weeks in a world war 2 bunker
  • Deprived of natural light
  • Results: all but one of the participants displayed a circadian rhythm between 24 - 25 hours
  • Siffre and the bunker study suggest that the natural sleep/wake cycle may be slightly longer than 24 hours but that it is entrained by exogenous zeitgebers associated with our 24 hour day
  • Folkard et al studied a group of 12 people who agreed to live in a dark cave for 3 weeks
    • going to bed when it was 11:45pm and waking up when it was 7:45am
    • The researchers gradually speeded up the clock so an apparent 24 hour day eventually lasted only 22 hours 
    • Results: only one of the participants was able to adjust to the new regime
    • This shows the existence of a strong free-running circadian rhythm that cannot easily be overridden by changes in the external environment
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Evaluation of circadian rhythms

  • There was some practical application to shift work. Circadian rhythms gives researchers better understanding of the consequences that can occur as a result of their disruption.
  • Night workers engaged in shift work experience a period of reduced concerntration around 6am meaning mistakes and accidents are more likely to occur
  • Research also suggests the relationship between shift work and poor health - shift workers are 3x more likely to develop heart disease which may be due to the stress of adjusting to different sleep/wake patterns and the lack of poor quality sleep during the day
  • Research into the sleep/wake cycle may have economic implications in terms of how best to manage worker productivity
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Evaluation of circadian rhythms

Practical application to drug treatments

  • Circadian rhythms co-ordinate a number of the body's basic processes such as heart rate, digestion and hormone levels
  • This can affect the action of drugs in the body and how well they are absorbed and distributed
  • Research  into circadian rhythms shows that there are certain peak times during the day or night when drugs are more likely to be most effective
  • This then led to the development of guidelines of when to take a drug during a day
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Evaluation of circadian rhythms

Practical application to drug treatments

  • Circadian rhythms co-ordinate a number of the body's basic processes such as heart rate, digestion and hormone levels
  • This can affect the action of drugs in the body and how well they are absorbed and distributed
  • Research  into circadian rhythms shows that there are certain peak times during the day or night when drugs are more likely to be most effective
  • This then led to the development of guidelines of when to take a drug during a day
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Evaluation of circadian rhythms

Use of case studies and small samples

  • The people involved in Aschoff and Wever / Siffre's study may not be representative of the wider population and this limits the extent to which meaningful generalisations can be made
  • Siffre conducted another study, at the age of 60, that his internal clock ticked much more slowly than when he was a young man. 
  • This shows that even with the same man involved, there are factors that vary which may prevent general conclusions being drawn

Poor control in studies

  • Even though the p's were deprived of natural light, they still had artifical light
  • In Siffre's study, he turned on a lamp everytime he woke up which remained on until he went to sleep. 
  • He assumed that arficial light has no effect on the free-running biological rhythm
  • However, Czeisler et al were able to adjust p's circadian rhythm from 22 to 28 hours using dim lighing
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Evaluation of circadian rhythms

Individual differences

  • Generalisations being made, when in fact peoples cycles can vary
  • Duffy et al revealed that some people display a natural preference for going to bed early and rising early
  • Some people choose to do the opposite
  • There are also age differences in sleep/wake patterns
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Evaluation of circadian rhythms

  • Studies that have deprived humans of natural light have still allowed artifical light, which may give many of the benefits of natural light - this reduces the validity of these studies as in Siffre's study
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