Biological Rhythms



  • patterns of changes in the body, conforming to cyclical time periods
  • influenced by endogenous pacemakers (internal body clocks)
  • influenced by exogenous zeitgebers (environmental stimuli)
  • ultradian rhythm: less than a day (multiple times)
  • circadian rhythm: roughly a day (24hrs)
  • infradian rhythm: greater than a day
  • circannual: roughly a year
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Circadian Rhythms

  • roughly 24 hours
  • Sleep/Wake Cycle and Core Body Temperature
  • CBT is lowest at 4am (36˚C) and highest at 6pm (38˚C) and affects cognitive performance

Sleep/Wake Cycle

  • feel drowsy during the night when it is dark, and alert when it is day and bright
  • daylight is an exogenous zeitgeber
  • Siffre: spent 2 months in an underground cave with no natural light until September (but believed it to be mid-August) - shows the influence of light on biological rhythm (settled just longer than 24hrs)
  • Aschoff and Wever: put participants in a WW2 bunker for 4 weeks with no natural light; circadian rhythm settled to 25 hours (apart from one) - suggests that SWC may be slightly longer than 24hrs, but is entrained by light (exogenous zeitgebers)
  • Folkard: put 12 people in a dark cave for 3 weeks and subtly fast-forwarded the clock so that each day was only 22 hours long; each participant except one struggled to adapt, suggesting the SWC is an innate circadian rhythm that can not be overridden 
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Circadian Rhythms Evaluation


  • Knutsson: found that shift workers were 3x more likely to develop heart disease (importance of circadian rhythms on body)
  • Boirin: found that shift workers were the most tired and made the most mistakes at 6am due to a lack of concentration at this time (circadian trough)
  • practical application to drug therapy (pharmacokinetics) - when drugs are taken at certain times of day, they are most effective (especially cancer and epilespsy medication - Baraldo)


  • Duffy: individual differences between 'morning' and 'night' people
  • research suggests that people have natural SWCs between 13 and 65 hours (not so strict)
  • Czeisler: found that they could shift participants' SWC between 22-28hrs with dim artifical light - questioning the internal validity of 'no-natural-light' studies
  • studies have small sample sizes or are singular case studies - are not generalisable
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Ultradian Rhythms

  • occurs multiple times a day
  • the sleep cycle (stage of sleep)

Stages of Sleep

  • 5 stages of sleep over 90 minutes, on a cycle
  • can be monitored using EEGs
  • Stage 1 + 2: sleep escalator; light sleep; easily woken; slow, rhythmic alpha-to-theta waves
  • Stage 3 + 4: delta waves; great amplitude; deep sleep; slow wave sleep; not easily woken
  • Stage 5: REM sleep (Rapid Eye Movement); paralysed body; fast brain activity; eye activity highly correlated to events in the dream
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Ultradian Rhythms Evaluation


  • Dement and Kleitman: found that REM activity was highly correlated with experience in their research of 9 sleeping adults - used EEGs (with caffeine and alcohol variables controlled) 
  • research has been done with EEGs - high scientific credibility
  • lots of replications of Dement and Kleitman's research has found the same thing (high external validity)
  • research has control over caffeine and alcohol 


  • small sample sizes of evidence
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Infradian Rhythms: Seasonal Affective Disorder

  • SAD
  • persistent low mood (depression)
  • disinterest in acitivity and life
  • shorter daylight hours
  • circannual and circadian aspects (seasonal and sleep-wake cycle)
  • melatonin secreted at night to help us sleep
  • more melatonin secreted over longer nights
  • melatonin affects serotonin production (more melatonin = less serotonin)

Evaluation [For]

  • phototherapy application (a lightbox that projects artifical light to reset melatonin levels)
  • Eastman: found that phototherapy gave an improvement of SAD in 60% of patients

Evaluation [Against]

  • Eastman: also found a 30% placebo effect
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Infradian Rhythms: The Menstrual Cycle

  • governed by hormonal changes which regulate ovulation
  • first day: womb lining is shed (with typical cycle 28 days - 22-35)
  • oestrogen stimulates egg production and its release
  • progesterone triggers womb lining to grow, and prepares womb and body for lining to shed
  • if preganancy does not occur, the egg is absorbed by the body and menstruation occurs
  • Stern and McClintock: took pheromones from armpits of 9 regular women (different cycle stages) (frozen and treated); given to 20 women with irregular periods (upper lips); (68%) cycles synched up to 9 women (their odour donors)

Evaluation [For and Against]

  • evolutionary theory support - women fall pregnant at the same time to raise children together
  • Schank: evolutionary theory is not correct (would cause greater competition)
  • many variables affect periods (not just hormones) - confounding variables for case studies
  • small samples of women in research
  • Trevathan: found very little evidence for synchronisation (low external support)
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Endogenous Pacemakers

Suprachiasmatic Nucleus (SCN)

  • collection of nerve cells in the hypothalamus (in each hemisphere)
  • master oscillator in mammals for the Sleep/Wake Cycle
  • lies just above the optic chiasm (from which it receives information about light)
  • allows biological clock to adjust to changing light patterns
  • Pineal Gland recieves information from SCN, releasing melatonin over night
  • DeCoursey: destroyed SCN in 30 chipmunks and found that most died when released into the wild, as their SWC was disrupted and they were active during vulnerable night hours
  • Ralph: mutant hamsters with a SWC of 20 hours had their Suprachiasmatic Nucleus cells injected into normal hamsters' brains; found the normal hamsters' SWCs shifted to 20 hours = biological body clocks are a big influence
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Endogenous Pacemakers Evaluation


  • animal studies are under controlled conditions 
  • animal studies can be replicated in large quantities (once it has passed ethical issues)


  • animal studies are unethical as there is no consent
  • animal studies are ungeneralisable to humans as humans are much more complex
  • evidence is from small sample sizes or case studies
  • Campbell and Murphy: there are other peripheral oscillators (not just the SCN), as they found a SWC shift after shining light on knee skin cells
  • Damiola: the liver is also a peripheral oscillator (saying SCN is the only one is reductionist)
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Exogenous Zeitgebers

  • external, environmental factors that reset biological clocks (entrainment)


  • affects SWC and other functions (blood circulation and hormonal secretion)
  • Campbell and Murphy: light detected in skin receptors of knees in 15 participants and their SWCs shifted by up to 2 hours

Social Cues

  • circadian rhythms start at 6 weeks old
  • circadian rhythms are entrained by 16 weeks
  • schedules imposed by adults are children's key influences
  • local times of eating and drinking train circadian rhythms and help to beat jet lag in adults
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Exogenous Zeitgebers Evaluation


  • Burgess: found that jet lagged people (travelled East to West) recovered better when exposed to continuous bright light - suggests that light has a strong influence over melatonin secretion and therefore circadian rhythms


  • Miles: SWC of a blind man was not shifted from 24.9 hours by social cues
  • those who live in the Artic have normal cycles despite the sun not setting during Summer
  • evidence is from small-scale studies and exceptional case studies
  • exogenous zeitgebers and endogenous pacemakers usually interact, and so separating them to study individually is hard and unrealistic
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