Endogenous Pacemakers

• 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

Basics

• 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

• 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

Basics

• 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

• 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

Basics

• 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!
• #YIPPEE

• 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

Basics

• 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.
• #TEAMWORK

• 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

Basics

• 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.
• #HAMSTERCRUELTY

• 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

Basics

• 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
• #SADFACE

• 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

Basics

• 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?
• #I'VEGOTTHEPOWER

• 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

Basics

• 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