Circadian Rhythms

• What is a Biological Rhythm

  distinct patterns of changes in body activity that conform to cyclical time periods.

  • can be influenced by internal body clocks (endogenous pacemakers) or external changes to the environment (exogenous zeitgebers)
• Circadian Rhythm


  a type of biological rhythm that operates on a 24 hour cycle

  this regulates a number of bodily processes e.g. sleep/wake cycle and core body temperature.
• Sleep/Wake Cycle

  • melatonin (produced in the pineal gland), causes drowsiness and lowers body temperature.
  • cortisol (produced in the adrenal gland), forms glucose or blood sugar to enable anti-stress and anti-inflammatory functions in the body.
  • circadian clock is located in the suprachiasmatic nucleus (SCN) in the hypothalamus.
  • SCN is a tiny area, containing 20,000 neurones, but has the responsibility for sending signals to other parts of the brain to regulate sleep/wake cycle, body temperature, hormone production and other functions.
• Core Temperature


  usually reaches its minimum around 4:30-5:00am
• Melatonin


  absent during daylight hours

  • typically begins to be produced around 8:00-9:00pm and stops around 7:00-8:00am.
• Sleepiness


  • sleepiest at 2:00-3:00am
  • shallower period around 2:00-3:00pm
• Siffre's Cave Study


  • Siffre spent several extended periods underground to study the effects of biological rhythms.
  • deprived of exposure to natural light and sound.
  • re-surfaced in mid-September believing it was mid-August.
  • performed a similar feat for 6 months in a Texan cave - 10yrs later.
  • his biological rhythm settled beyond 24hrs (around 25hrs) though he did continue to fall asleep and wake up on a regular schedule.
• Other research

  Aschoff and Wever convinced a group of ppts to spend 4 weeks in a WW2 bunker deprived of natural light.

  • all but one ppt (whose cycle extended to 29hrs) displayed a circadian rhythm of 24-25hrs.
  • both studies suggested that the sleep/wake cycle may be slightly longer than 24hrs but is entrained by exogenous zeitgebers associated with out 24hr day.

  Folkard et al studied a group of 12 people living in a cave for 3 weeks, sleeping at 11:45pm and waking at 7:45am.

  • researches gradually speeded up the clock to 22hrs.
  • not one of the ppts was able to comfortably adjust to the regime.
  • strong free-running circadian rhythm can't easily be overridden by changes in the external environment.
• Strength - Application: Pharmacokinetics

  • by understanding circadian rhythms and their impact on health, it can help determine the best time to administer drug treatments.
  • e.g. the risk of heart attack is greatest in early morning so drugs can be taken at night, but not released until when they will be most effective at dusk.
  • there are peak times for administration and dosage of a variety of drugs that treat a range of disorders e.g. cancer, epilepsy, respiratory issues, that increase their efficacy.
• Weakness - Poor Control in Studies

  • in cave studies the exposure to artificial lights such as torches/phones wasn't controlled.
  • was assumed it was only natural light that has an influence on our biological rhythms, however research by Czeisler found that dim artificial lighting could adjust the circadian rhythm between 22-28hrs.
  • results of the original studies may lack validity and sleep wake cycle may vary considerably more.
  • additionally, the impact of artificial lighting on our circadian rhythms also had practical implications when it comes to the use of electronic devices such as iPad and smartphones during different times of the day.
• Weakness - Case Study Evidence

  • Siffre's study was only on 1 individual .
  • Folkard, Wever and Aschoff only a handful of people.
  • impossible to generalise these results to whole populations of people, as there may be some individual differences in research sample that makes these ppts respond the changes the sleep/wake cycle in atypical ways.
  • e.g. Siffre noted that when we went into the cave at 60yrs, his body responded differently, body clock followed more of a 48hr cycle.
  • Czeisler has found evidence that cycles can vary by as much 13-65hrs.
  • suggests that results can't be generalised from these small samples onto everyone, as factors like age and gender may have significant impacts on our circadian rhythms.
• Weakness- Application to Shift Work

  • shift work has been found to lead to desynchronization of circadian rhythms and can lead to adverse cognitive and physiological effects.
  • research has shown that night shift workers suffer a concentration lapse at 6am, increasing the likelihood of accidents, and shift workers are also 3 times more likely to suffer from heart disease as a result from stress of adjusting to sleep/wake cycles.
  • research has many economic implications, in terms of maintaining worker productivity and preventing accidents in the workplace.