circadian rhythms

• Circadian Rhythms


  One biological rhythm is the 24-hour circadian rhythm ('body clock'), which is reset by light
• Examples of circadian rhythms

  • Sleep-wake cycle
  • Body temperature
  • Hormone production
• Sleep-wake cycle


  An example of a circadian rhythm, which dictates when humans should be asleep and awake
• Light


  Provides the primary input to the circadian system
• Circadian rhythm regulation


  1. Light is detected by the eye

  2. Messages sent to the suprachiasmatic nuclei (SCN)

  3. SCN coordinates the activity of the entire circadian system
• Sleeping and wakefulness

  Not determined by the circadian rhythm alone, but also by homeostasis
• Endogenous Pacemakers

  Internal mechanisms that govern biological rhythms (e.g. sleep-wake cycle)
• Suprachiasmatic Nucleus (SCN)


  The most important endogenous pacemaker, closely linked to the pineal gland
• SCN


  • Receives information about light (exogenous zeitgeber) via the optic nerve
  • Sends signal to pineal gland
• Exogenous Zeitgebers


  Environmental events that are responsible for resetting the biological clock of an organism
• Exogenous Zeitgebers


  • Social cues such as meal times
  • Light which is responsible for resetting the body clock each day
• Low levels of light

  Exogenous zeitgeber
• SCN and Pineal Gland

  Endogenous pacemakers
• Exogenous zeitgeber effect on sleep

  1. Low levels of light (retina)

  2. Melanopsin carries signals to SCN

  3. SCN sends signal to the pineal gland

  4. Secretion of melatonin

  5. Induced Sleep
• Siffre (1975) found that the absence of external cues significantly altered his circadian rhythm
• Aschoff & Weber found that participants in a bunker with no windows and only artificial light settled into a longer sleep/wake cycle of between 25-27 hours
• Duffy et al. (2001) found that 'morning people' prefer to rise and go to bed early whereas 'evening people' prefer to wake and go to bed later
• Morgan (1955) bred hamsters so that they had circadian rhythms of 20 hours rather than 24. SCN neurons from these abnormal hamsters were transplanted into the brains of normal hamsters, which subsequently displayed the same abnormal circadian rhythm of 20 hours
• Nous


  Biological clock
• Pacemakers


  Biological clocks that regulate circadian rhythms
• Suprachiasmatic nucleus (SCN)


  Part of the brain that acts as the body's master biological clock
• Endogenous Pacemakers

  1. Morgan (hamsters)

  2. Issues with animal research
• Exogenous Zeitgebers


  1. Outline exogenous zeitgebers

  2. Social cues

  3. Light
• Research Support


  1. Siffre

  2. Issues with case study method
• There is research support for the importance of endogenous pacemakers, in particular the SCN, in relation to the sleep-wake cycle
• Morgan

  • Bred hamsters with 20 hour circadian rhythms, transplanted SCN neurons into normal hamsters who then displayed 20 hour rhythms
• Research on animals may be unable to explain the role of endogenous pacemakers on the sleep-wake cycle in humans
• Siffre's case study has been the subject of criticism due to potential researcher bias and lack of generalisability
• Lack of natural light


  Disruption of biological (circadian) rhythms
• Aschoff & Weber


  • Studied participants living in a bunker with no windows and only artificial light, found they settled into a 25-27 hour sleep/wake cycle
• Research examining endogenous factors carried out on animals may be unable to explain the disruption of biological rhythms in humans
• Humans would respond very differently to manipulations of their biological rhythms compared to animals due to biological and environmental differences
• The research could be criticised for being reductionist as it only considers a limited range of factors and fails to consider other viewpoints
• The disruption of biological rhythms could also be the result of other factors like illness or medication, not just light/SCN