Exogenous Zeitgebers

Exogenous Zeitgebers - AO1 and AO2

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AO1 - Light

  • Light is the dominant zeitgeber in humans
  • Can reset the SCN and other oscillators throughout the body (the protein cryptochrome - CRY - is light-senstitve)
  • Campbell and Murphy: if you shine a light on the back of participants' knees, their circadian rhythms will shift


  • Basically, light is the most important thing for helping set the old internal rhythms!
  • This includes the SCN and other oscillators, such as a protein amusingly shortened to CRY, because CRY is what you'll do if you don't remember this in the exam.
  • Campbell and Murphy found that even shining a light on the back of someone's knees does the trick - which is odd, but memorable.
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AO1 - Social Cues

  • Thought to be the main zeitgeber up until recently
  • We eat meals, wake up and go to bed at socially determined times: our daily rhythms appear to be entrained by convention, not biology
  • All parts of the body produce their own oscillating rhythms and some of these are not primarily reset by light
  • Davidson: the zeitgeber for cells in the heart and liver is likely to be mealtimes as they are reset by eating


  • "What do you MEAN, this isn't the most dominant zeitgeber!" - a psychologist who thought it was
  • Social cues do look, on the surface, like they're the most important factor - almost everything we do is based around seemingly socially-determined times
  • But there are some pesky oscillators that are not reset by light like everything else.
  • E.g. the heart and liver, which are reset when you eat.
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AO1 - Temperature

  • Biological rhythms can be entrained by temperature
  • Leaves on deciduous trees change colour and drop off because of changes in temperature and day length
  • This is also a factor in the onset of hibernation
  • Lopez-Olmeda: temperature is the second most dominant zeitgeber


  • Temperature can have an effect on biological rhythms and all!
  • This is most often seen in the natural world, with autumn leaves and hibernating creatures...
  • ...but Lopez-Olmeda says that temperature is the second most dominant zeitgeber in humans after light, and they know more about this type of thing than you or I ever will...
  • ...so you might as well take their word for it.
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AO2 - Kate Aldcroft Cave Study

  • This undermines the exogenous zeitgebers explanation
  • Aldcroft spent 25 days in a cave
  • Her body temperature followed a 24-hour rhythm, yet her sleep rhythm was on a 30-hour cycle
  • Separate oscillators in her body were working independently
  • Highlights the interactive nature of EPs and EZs, because the rhythms desynchronised without external cues


  • I went through this very study in my Endogenous Pacemakers set
  • I'm not typing it all out again
  • Look through that if you want more detail, but if I just write this: "#TEAMWORK"
  • Does that remind you of anything?
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AO2 - Dim Lighting

  • This supports the exogenous zeitgebers explanation
  • Czeisler et al: altered participants' rhythms using just dim artificial lighting
  • Previously, it was not thought this could be done
  • Shows that even artificial or dim lighting can be an effective zeitgeber


  • This Czeisler guy and his crack team of dimmer-switch operators found that even dim artificial lighting could reset participants' rhythms
  • This shows that zeitgebers can be subtle and still have an effect
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AO2 - Infradian Rhythms

  • This supports the exogenous zeitgebers explanation
  • Russell et al: placed an odour on the upper lip of 11 women, 3 times a week, over 4 months
  • Extracted from the armpit of a female donor, with a history of a very regular menstrual cycle
  • She was not permitted to wash under her arms, and the participants were not permitted to wash their faces for 6 hours after having the odour placed on them
  • The participants' menstrual cycle became synced with the donor's
  • Shows that pheromones from one woman can influence another, making them a type of exogenous zeitgeber
  • However, this is a highly unlikely scenario to happen in real life, so it may well lack ecological validity


  • Don't make me do this. Y'all can read.
  • #GROSS
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AO2 - Social Customs

  • This supports the exogenous zeitgebers explanation
  • Social customs can help to entrain biological rhythms, e.g. when shifting time zones
  • E.g. eating and sleeping when the clock says, not just when you want to
  • Fuller et al: suggests 1 day of fasting followed by 1 day on the new system can help entrain your biological rhythms to the new country's time


  • Social customs as a zeitgeber do have real-world usages!
  • When travelling to a country with a different time zone, you need to follow the time there, not your own personal body clock...
  • ...or, worse still, the time from your own country.
  • Fuller et al, with all their magical jet-lag wisdom, say that 1 day of fasting followed by 1 day on the new country's time will help entrain your rhythms before you go.
  • #HANDY
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AO2 - Real-World Applications

  • This supports the exogenous zeitgebers explanation
  • Stevens: found evidence of increased rates of breast cancer among women in well-lit societies (link to melatonin)
  • We can, from this, develop strategies to decrease these rates or develop treatments


  • Yippee! MOAR psychology in the real world.
  • Stevens has found that women in well-lit socities show an increased rate of breast cancer, presumably because they don't have enough melatonin
  • This info can help people find ways to bring those numbers down.
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