circadian rhythms

• What is a circadian rhythm with an example?


  Biological rhythms that last for around 24 hours such as the sleep/wake cycle
• What else is the sleep/wake cycle under?


  Homeostatic control. The homeostatic need for sleep increases through the day as you are awake for long periods of time
• What is the circadian rhythm intolerant of?


  Any major alterations in sleep-wake cycles (e.g. as a result of jetlag)
• What does SCN stand for and what is it?


  The Suprachiasmatic Nucleus which is a small group of brain cells that act as the main endogenous pacemakers.
• What does the SCN trigger from the pineal gland?


  The SCN triggers the pineal gland to release melatonin
• What is melatonin?


  Hormone produced by pineal gland when to reduce optic nerve activity (reducing brain activity) as night falls, making a person sleepy.
• What happens to melatonin production as daylight levels increase?


  Melatonin production stops completely
• What is entrainment?


  The endogenous pacemakers synchronise with the exogenous factors.
• Examples of entrainment?


  • Bedtime routines
  • mealtimes
• Strengths to circadian rhythms?

  1. Application to drug treatments:

  • Pharmacokinetics (action of drugs on the body)
  • Certain peak times at day or night which is when drugs are more likely to be most effective
  • Led to the development of guidelines regarding timing of drug dosage (Baroldo 2008)
• What are the weaknesses of circadian rhythms

  1. Shift Work:

  • Night workers tend to experience more mistakes around 6am due to reduced concentration (Boivin et al 1996)
  • Shift works are 3x more likely to develop heart disease - this could be due to stress of adjusting sleep/wake cycles and poor quality of sleep during the day (Knutsson 2003)

  1. Use of case studies and small samples:

  • Issues of generalisation - it is not representative
  • Siffre reentered cave aged 60 when internal clock = slower so no certainty when drawing conclusions
• What was the Decoursey at al (2000) study?


  • Destroyed the SCN of 30 chipmunks and then returned them to their natural habitat and observed them for 80 days
  • FOUND that their sleep/wake cycle disappeared which led to a significant proportion being killed as they were vulnerable
• What did Ralph et al (1990) study?

  • The SCN is transplanted from mutant hamsters with 20 hour sleep-wake cycle into the brains of normal hamsters
  • This led to the normal hamsters also defaulting to 20 hour sleep-wake cycles
  • Could help those with insomnia to get into a regular cycle
• What did Campbell and Murphy (1988) study?


  • 15 sleeping participants were woken up during the night at a series of intervals and light was shone at the back of their knees
  • This disrupted the pps's sleep schedule up to 3 hours - SUPPORTS the role exogenous zeitgebers have in controlling biological rhythms
  • Can be used to help prevent jetlag or help them adjust to new schedules
• What did Siffre (1975) study?


  • Aim - establish the role of light as a zeitgeber in the sleep/wake cycle
  • Siffre spent 6 months underground in dim light and his bodily functions were monitored
  • Bodily functions were erratic at first and generally had a cycle of 25 hours
  • However, sometimes cycle would drastically change to 48 hours
  • Conclusion - pattern of sleep/wake cycle remained even without external cues