Circadian Rhythms



Biological rhythms- cyclical behaviours, repeated periodically, controlled by edogenous pace makers (internal biological clocks) and exogenous zeitgebers (external cues, light

Circadian Rhythm- biological cycles lastiing 24 hours, sleep wake cycle which is facilitated by time clocks and external cues (meal times- regular events)

Main biological clock- located in hypothalumus, named the superchiasmatic nuclus (inbuild circadium pattern) when damaged in rats, the circadian rhytm is ditruped ie sleep (Zucker et al). SCN regulates the production of melatonin in pineal gland (endogenous pacemaker) alsol connect to the retina of the eye through seperate pathway. shows a indirect link betweenn exogenous zeitgebers and production of melatonin- pineal gland and SCN work together to mainntain rhythm. light can reach the brain via other roots. Cambelle- shined lightof back of knees, which shows other osallators exsist throughout the body maintaining the circadian rhthym through the use exogenous zeitgebers.

core body temperature- example of circadian rhythm- lowest at 4:30am 36 degrees and highest at 6:00pm at 38 degrees. decline at lunch and dip even without food

Hornmones- cortisol is lowest at midnight and peaks at 6am. makes us alert,. explain why we struggle ot think clearly at 4am  if woken. growth hormones and melatonin peaks at midnight

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AO2- Aschoff and weaver

Supports the seep wake cycle and internal body clock.

  • participants placed in a WW2 bunker with no external cues
  • they found that participants had circadian rhythms of 24-25 hours sometimes up to 29

This demonstrates the exsistance of endogenous pacemakes which persists without exogenous zeitgeber to influence them .

the results also showed the importance of external cues as the internal cues were not accuate without them

However Study can be critised-

  • partcipant volunteered- more motivated and mre aware of being monitored- demand characteristics
  • small sample- caant generlaise- non- representative to the target population.

lowers the reliability of the results- unable to give reliable results for circadium rhythm

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AO2- Siffre

Supports the importance of endogenous pacemakes

  • spent 6 months in a cave with no external cues
  • found his circadium rhythm reanged from 25 to 30 hours

highlights the exsistance of internal circadium rhythm- around normal

also shows the important of exogenous zeitgebers inn regulatinng internal clock


  • case study- single case study invovling one individual and such findings are not genralisable to other individuals due to indivdual differences

lowers the reliability of the results- unable to give reliable results for circadium rhythm

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AO2- Duffy et al

Found a case for individual differnces

  • found that morning perople preferred to rise early and go the bed early
  • whereas evening people prefer going to sleep and wakeing up later

showing that people circadian rhythsm could vary from one to another

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AO2- Folkard et al 1977

found a link between temperature and changes in cognitive performance

  • looked at learning ability of 12-13 year old children who had stores read to them at either 9:00am or 3:00pm.
  • after one week the afternoon score (high core body temperature) showed supior recall and comphrension and retained 8% more material

suggesting long term recall is best when body temperature is high  

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AO2- Giesbretch et al 1993

Evidence to support that temperature does cause changes in cognitive perofrmance

  • place participants in cold water and asked to do tasks
  • found that their cognitive ability was worse

this supports the link between the affect of temperature and cognitive performance

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IDA- real world application


  • study of how timing affects drug treatment and as circadian rhythm affects digestionn, heart rate and hormones, this can be taken into account when taking drugs

for example

- medicine which affect certain hormones should be taken when that level of hormone is high not low as you wont get an effect and is more efcective when the hormone is high

- asprin should be taken in the evening when treating heart attacks as it is most effective as heart attacks mainly happen in the early hours of the morning  

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IDA-biologcal explanation

  • behaviour is explained in terms of biological structures
  • it is much more complex than just this- over simplifies
  • it is theefore not so deterministic as the biological approtch suggests
  • even though nature is a strong influence, envronmental factors and exogenous zeitgebers have clearly also got strong role in controlling biologcal clocks
  • shown in the studies of sifree and aschoff and wever
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