Circadian rhythms

• Circadian rhythms


  A type of biological rhythm where cycles are 24 hours in length.

  
  

  Regulate a number of body processes such as the sleep/wake cycle and changes in core body temperature.

  
  

  The sleep/wake cycle dictates when humans and animals should be asleep and awake.
• Exogenous zeitgeber


  EZ - light

  
  

  Light is detected by photoreceptors in the eye, which sends message about the level of brightness to the SCN. The SCN then uses this information to coordinate the activity of the entire circadian system.

  
  

  For example, it regulates the release of melatonin by alerting the pineal gland, which induces feelings of sleepiness.

  
  

  Changes in light exposure can trigger the desynchronisation of a 'pre-set' sleep-wake cycle.
• Endogenous pacemaker


  EP - melatonin

  
  

  Levels of melatonin peak during the hours of darkness - activates chemical receptors in the brain to encourage feelings of sleep.
• Role of homeostasis


  The sleep/wake cycle isn't determined by the circadian rhythm alone, but also by homeostasis (regulation of internal conditions in the body)

  
  

  When an individual has been awake for a long time, homeostasis tells the body there's a need for sleep because of energy consumption.

  
  

  This homeostatic drive for sleep increases throughout the day, reaching its maximum in the late evening.
• Core body temperature


  Another circadian rhythm.

  
  

  The core body temperature also cycles along with the sleep-wake rhythm, decreasing during the nocturnal sleep phase and increasing during the wake phase repeatedly in a 24-hour circadian rhythm.

  
  

  Sleep typically occurs when the core temperature starts to drop. Core body temperature then starts to rise towards the end of the sleep cycle to promote feelings of alertness first thing in the morning.
• Siffre (study)


  Siffre was studied by Dement when he spent 61 days in an underground cave.

  
  

  When he returned home, he believed the date to be a month earlier than it actually was.
• Siffre (findings)

  This suggests his 24 hour sleep cycle was increased by the lack of external cues, making him believe one day was longer than it actually was.

  
  

  The absence of external cues had significantly altered his circadian rhythm. This demonstrates that how changes in light exposure can trigger the desynchronisation of a 'pre-set' sleep-wake cycle.
• Siffre (conclusions)

  This also demonstrates a disconnection between psychological time and the clock, suggesting that there is 'an internal clock dependent of the natural terrestrial day/night cycle'.

  
  

  A 'free-running' circadian rhythm

  ie: one that isn't affected by exogenous zeitgebers.
• Aschoff and Weaver (study)


  Aschoff and Weaver later studied free-running circadian rhythms.

  
  

  They studied a group of participants living in a WWII bunker for 4 weeks, deprived of natural light.
• Aschoff and Weaver (findings)

  Having been isolated from environmental cues, they found that 'all subjects showed free-running circadian rhythms' - showing an increased sleep-wake cycle between 24-25 hours.
• Aschoff and Weaver (conclusions)

  These findings demonstrate that although the free-running circadian rhythm is longer than 24 hours, there are specific exogenous zeitgebers which entrain the system to conform to a 24 hour cycle.
• A03 - Individual differences (P)

  Circadian rhythms may not always have to conform to cyclical 24 hour periods.
• A03 - Individual differences (E)

  For example, a professor of sleep medicine at Oxford Uni delayed the starting time of a High School to 10am, instead of the usual 8am.
• A03 - individual differences (E)

  The researchers noted that the GCSE results went up (19% increase of pupils scores 5 A*-C grades).
• A03 - individual differences (E)

  Genetic screening has supported that in adolescence the sleep-wake cycle is set to wake up later and go to bed later than in adulthood.
• A03 - individual differences (L)

  This is a real-life example of how the circadian rhythms of teenagers aren't always in line with that of adults, and so an appreciation of this can improve educational attainment.
• Duffy et al


  Found some people had natural preference for getting up and going to bed early (morning people) and others prefer to wake and go to bed later.

  
  

  This demonstrates that there may be innate differences in circadian rhythms, which suggests that researchers must focus on / take these into account during investigations.
• A03 - real life applications (P)

  Gives researchers a better understanding of the consequences that can occur as a result of disruption to circadian rhythms.
• A03 - real life applications (E)

  According to research, shift workers are 3x more likely to develop heart disease which may be in part due to the stress of adjusting to different sleep/wake patterns and having a lack of good quality sleep during the day.

  
  

  This suggests that there may be practical uses of an improved understanding of the effects of desynchronisation.
• A03 - real life applications (E)

  Also, this has economic implications.

  
  

  Research found that night workers engaged in shift work experience a period of reduced concentration around 6 in the morning - mistakes and accidents during this time are more likely.
• A03 - real life applications (L)

  So, research into circadian rhythms can advise companies who employ shift-workers on how to best manage worker productivity and ensure a good health and safety record in the workplace.