circadian rhythms

• sleep-wake cycle


  • happens every 24 hours
  • research has attempted to investigate how our rhythms are controlled
• all bio rhythms seem to be controlled and regulated in 2 ways

  1. endogenous (internal) pacemakers (EPs):

  • these refer to internal controls
  • for example, within the brain (body clock)

  1. exogenous (external) zeitgebers (EZs):

  • external factors, usually environmental or social
  • e.g. light and dark, temperature, seasons, movement from other people, alarm clock
• case studies - siffre study

  siffre study:

  • one way of investigating importance and roles of exogenous and endogenous factors is to remove all obvious exogenous factors (all cues to time) and see what effect it has

  first person to do this was in a systematic way was michel siffre

  • he spent 7 months in an underground cave with no cues at all to time; no watches, clocks or televisions
  • he was monitored via computers above ground that recorded all of his bodily functions and avtivities
  • at first, sleep-wake cycle was fairly irregular but it eventually settled into one of roughly 25 hours
• case studies - siffre study - evaluation

  evaluation:

  • high validity
  • lacks reliability
• case studies - aschoff study (experiment)

  aschoff and weaver:

  • used underground bunker
  • student Ps therefore had no cues to light and dark, and could select their own light-on active or light-off sleep periods
  • similarly to siffre, the Ps settled into a regular sleep-wake cycle, however it extended to between 25-27 hours
  • therefore, revealing that circadian rhythms persist even in the absence of light/dark cues but that the cycle tends to drift towards 25 hours
• case studies - aschoff study (experiment) - evaluation

  evaluation:

  • more scientific than siffre
  • higher reliability than siffre
• case studies - evaluation

  evaluation:

  • the case studies showed that our body rhythms are able to carry on in a regular cycle without any external cues, we need environmental cues to maintain a regular 24 hour cycle
  • otherwise, our cycles become 'free-running' and become desynchronised with the outside world
• how do EPs and EZs work together?

  • light receptors located in the eye (and other parts of the body) sense when it is dark
  • this sensory info then travels via the optic nerve to the hypothalamus
  • a specific part of the hypothalamus called the suprachiasmatic nucleus (SCN) then instructs the pineal gland to start to secrete melatonin (makes us feel happy)
  • the body continues to produce melatonin until the light receptors sense light at which point the hypothalamus instructs the pineal gland to stop melatonin production and serotonin is produced (makes us feel alert)
• suprachiasmatic nucleus (SNC)


  the SCN is located in the hypothalamus

  • it is a bundle of nerves with an inbuilt circadian rhythm
  • this is particularly important endogenous pacemaker
  • evidence for this comes from studies in which the SCN has been cut in hamsters to result in disrupted circadian rhythms
• animal studies - zucker

  zucker:

  • in 1 study, the optic nerve was cut, so hamsters hypothalamus was unable to register when it was light or dark
  • whilst it did still maintain a sleep-wake rhythm, it became slightly extended
  • in another variation, they damaged the SCN in the hamster, and then monitored their testosterone production and whether it was in line with breeding season
  • the found that the hamsters produced testosterone all year round (which they shouldnt) and the breeding cycle was completely disrupted
• animal studies - zucker - conclusion/evaluation

  zucker - conclusion/evaluation:

  • the findings clearly demonstrated that without their SCN functioning properly, they were unable to maintain regular biological rhythms, and therefore EPs are essential