Endogenous Pacemakers

Endogenous Pacemakers - AO1 and AO2

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AO1 - The Suprachiasmatic Nucleus (SCN)

  • Lies in the hypothalamus just above the optic nerve
  • Obtains information about light from the eye via the optic nerve
  • If our endogenous clock is running slow, morning light automatically shifts the clock ahead, putting the rhythm in step with the world outside
  • Ventral SCN = quickly reset by external cues
  • Dorsal SCN = more resistant to being reset


  • There's this thing in your hypothalamus called the SCN that's near the optic nerve
  • It gets information about light from the eye
  • If our body clock is not in sync with the world outside, the morning light puts it back in order
  • There's the Ventral SCN, which is reset quickly, and the Dorsal SCN, which needs more convincing
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AO1 - The Pineal Gland and Melatonin

  • SCN sends signals to the pineal gland, directing it to increase production of melatonin at night
  • Melatonin induces sleep by inhibiting the brain mechanisms that promote wakefulness


  • Remember that SCN thing? Well here it is again!
  • This time it's sending messages to this thing called the pineal gland and telling it "Hey! It's night now! Get to work on making that melatonin!"
  • What I think it's getting at is that melatonin is not some magical sleep potion that just knocks you flat out - it just tells the parts of your brain that keep you awake to stop keeping you awake
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AO2 - Michel Siffre's Cave Study

  • This supports the endogenous pacemaker explanation
  • Siffre spent time in an underground cave with no external cues to guide his rhythms: he just ate and slept when he felt like it
  • His natural circadian rhythm eventually settled down to 24 hours
  • This shows that, even in the absence of external cues, a rhythm is maintained


  • OK! This crazy old guy called Michel Siffre decided "Hey - I'm going to live in a dark, empty, clock-less cave for a few months to study my internal rhythms!"
  • (God only knows how he was ever able to relate to another human again but hey ho)
  • Anyway, his circadian rhythm did run to 24 hours, even in this dark, empty, clock-less hell hole
  • This suggests that the endogenous pacemaker is, indeed, a thing, and can run without light, people or clocks to guide it!
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AO2 - Kate Aldcroft's Cave Study

  • This undermines the endogenous pacemaker 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
  • This highlights the interactive nature of EPs and EZs, because the rhythms desynchronised without external cues


  • Wahey! Another crazy cave-dweller!
  • This time, Kate Aldcroft found that she had different rhythms operating on different time frames!
  • This suggests that separate parts of her body were working independently, the rebels.
  • This shows that endogenous pacemakers and exogenous zeitgebers (don't worry, we'll get to those another time) need to work together to keep our rhythms in check.
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AO2 - 'Mutant' Hamsters

  • This supports the endogenous pacemaker explanation
  • Morgan bred 'mutant' hamsters so that they had circadian rhythms of 20 hours instead of 24
  • Their SCNs were then transplanted into normal hamsters
  • The normal hamsters then displayed the mutant rhythms
  • Provides clear evidence for the SCN as a factor that altered the circadian rhythm


  • Animal lovers look away now - category 4 animal testing approaching
  • (Category 4 of my list of categories that don't exist)
  • Morgan genetically engineered hamsters to have 20-hour cycles instead of 24-hour ones
  • Then - and this is the awful bit - transplanted their SCNs (hello darkness my old friend) into some normal hamsters, who then displayed these mutant rhythms
  • (Attack of the Mutant Hamsters - coming to a cinema near you this summer)
  • This shows quite painfully clearly that the SCN is an important factor in altering the circadian rhythm.
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AO2 - Blind Man Study

  • This undermines the endogenous pacemaker explanation
  • Miles studied a young man who had been blind from birth and had a circadian rhythm of 24.9 hours
  • He was exposed to various exogenous zeitgebers including clocks and social cues, yet found great difficulty reducing his internal pace
  • He had to take stimulants in the morning and sedatives in the evening just to keep his circadian rhythm on track
  • The fact that he could not adjust to the environment without EZs suggests that EPs alone are not enough to keep the circadian rhythm going


  • This poor guy has been blind from birth and has a slightly out-of-sync circadian rhythm
  • He cannot see light, time or people, so he cannot be susceptible to those cues, and has to resort to drugs just to function like a normal human
  • This suggests that his endogenous clock alone is not enough to keep the rhythm on track
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AO2 - ACTH Study

  • This undermines the endogenous pacemaker explanation
  • Born et al found that people who were told to wake up earlier than usual had higher levels of the stress hormone ACTH at this time
  • This hormone also contributes to the waking-up process
  • As the request to wake earlier had affected participants' hormone levels, this clearly shows that the process may be affected by our free will


  • Born et al told a bunch of people to wake up earlier than they norrmally would (psychologists must just hate small rodents and sleeping people)
  • These people were found to have a lot of the stress hormone ACTH - partially because it's a pain to wake up at stupid o'clock, but mostly because it also contributes to the waking-up process
  • This shows that we do have the power, to an extent, over our biological clocks, so they can't be totally controlled by our internal pacemakers now can they?
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AO2 - DeCoursey's Chipmunk Studies

  • This supports the endogenous pacemaker explanation
  • DeCoursey removed the SCNs from 30 chipmunks, then put them back into their habitats
  • They were promptly killed by weasels, because they had remained awake in their burrows where they could be heard
  • The death of the animals highlights the adaptive nature of sleep and the importance of the SCN in maintaining the rhythm to ensure sleep occurs at the best time


  • Remember what I said about small rodents and sleeping people? Well it still holds. Animal lovers look away now - category 5 animal cruelty approaching (CODE RED)
  • DeCoursey decided to sacrifice some chipmunks in the name of science
  • He? She? did this by removing their SCNs before putting them back into their habitats
  • This made their sleep rhythms go disastrously wrong - they stayed awake rustling around in their burrows and were heard by weasels (chipmunks gotta burrow, weasels gotta eat amirite?)
  • This shows the importance of the SCN in helping sleep to occur at times when you won't get eaten - should you happen to be a chipmunk that is
  • #RIP
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this is so helpful at explaining and simplifying things! thanks



Wow, your notes are incredible! What grade did you get?

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