What are the main theories of why we need to sleep?
- The washing machine hypothesis: clears waste products
- Restoration: benefits to immune system and wound healing
- Energy conservation and Protection: many animals are adapted for hunting/foraging at night or during the day (not both).
- Ontogenetic hypothesis: REM sleep is important for the developing brain (babies have mostly REM sleep and tons of it)
- Homeostasis: Rebalancing synaptic connections to prevent the cumulative effects of LTP
- Memory: you can’t store information while you are “actively” using a neural circuit
What is the main argument for sleep being useful for bodily cleaning?
- Siegel (05) suggests that free radicals are high in the chemicals produced from metabolism.
- They are highly reactive and oxidising, and can damage cells through binding; oxidative stress.
- In SWS, the lowered metabolism allows destruction of free radicals, preventing their bad effects. Prolonged sleep deprivation increases free radicals and oxidative stress.
What is the evidence for sleep being for restoration?
- Sleep deprived rats eventually die.
- Studies show insufficient sleep alters established cardiovascular risk factors to being more linked with cardiovascular morbidity.
- Unclear what exactly caused death, their brains looked normal and there was no obvious damage to other organs.
- Stress hormone levels were not unusually high.
- Also seen in people with 'fatal familial insomnia' in which they suffer from progressively worsening insomnia, and experience memory loss, autonomic dysfunction and prominent weight loss.
- If sleep is for repairing what physical activity has done, people who exercise more should sleep more.
- BUT Experimentally shown to not be the case. BUT BUT cognitive exercise is related to sleeping more.
- It has been suggested main function of SWS is to allow brain to rest/recover from daily activity. Has indeed been shown that those who have had long day of cerebral exericse then spend longer in Slow wave sleep.
What is unihemispheric sleep?
- One brain region 'shut down'
- Cannot do REM sleep
- Birds, aquatic animals, reptiles do this
- Seals have unihemispheric sleep in the water, but not on land
How do antidepressants challenge the REM sleep memory hypothesis?
- Antidepressants that block REM sleep do not impair memory
- Dolphins are smart, and they don't REM sleep
- BUT there is a REM rebound after you stop taking them, so it's needed for something
How do smells help our understanding of sleep?
- Associate a smell with a task
- Let patient sleep
- Present the same odour whilst they are in deep sleep, biasing their dreams
- When they wake up their performance is like 10% better on the task!
- This is only beneficial during slow wave sleep though, not REM
What is slow wave sleep?
- Little brain activity
- Not REM, probably not dreaming
How is SWS associated with memory?
- Memory is improved in p's given stimulation increasing SWS
- Significantly higher than in sham stimulation
How is sleep thought to affect the hippocampus?
- Hippocampus records cortical patterns
- During sleep the hippocampus 'plays back' patterns, transferring into the neocortex (and the LTM)
- HM bilateral removal of area including hippocampus, could not form new autobiographical memories
- Suggests that both the hippocampus is important for storing new memories, and that damage to it does not affect old ones
Why did John O'keefe win the nobel prize (again! Get over it UCL jeez)
- Individual hippocampal neurons fire dependent on location
- Called place cells, each has a preferred space
- The sequential pattern of activity lets you predict where the rat is moving
- Let rats sleep and continued recording
- Same pattern
- It dreamt of running the maze FUKIN MENTAL LAD
- Also replayed in the rest of the brain after a slight replay. ALMOST AS IF IT'S BEING TRANSFERRED
What happens when 'replay' is disrupted?
- Measure brain activity and look for 'sharp wave ripple' (the moment the replay happens)
- When this happens, disrupt the memory by waking the rat or something
- Memory gets much worse if you do this
What happens when replay is manipulated?
- Train rat to run left or right dependent on the pitch of a sound heard
- Reward equally for both
- Let them sleep
- Look at brain activity in sleep and during the task
- Play the high pitched sound
- Starts replaying the memory of running to the left (same activation)
- Look at human behaviour, same findings
- Associate pictures with sounds (cat meowing)
- 25 pairings (cued sounds) are presented during slow wave sleep, and all the stimuli are tested upon wake
- Performed better on the ones they had heard the sounds for
- More replay is happening!
Does sleep loss lead to reduction in homeostasis ability
- Yes and no
- Horne (78) did a meta-analysis. Found sleep deprivation did not interfere with ability to perform physical exercise. Also, no evidence of physiological stress response to sleep deprivation.
- However, sleep deprivation makes you less resilient to stress, even partial sleep loss leads to increased cortisol levels. This delays recovery of the hypothalamo-pituitary-adrenal axis, which interferes in glucocorticoid feedback and therefore leaves the individuals more prone to stress (Leproult, Copinschi & Buxton, 97).
- Sleep deprivation increases the homeostatic drive to sleep, resulting in changes in proinflammatory cytokines and glycogen levels.
- Lack of sleep = more severe physiologic, neurobiologic and behavioural consequences.
What is Allostatic load?
- "Wear and tear on the body" which grows over time with exposure/repeated stress.
- Can explain how frequent activation of stress response can damage bodylong-term. So long-term, SD can be involved in hypertension, increased proinflammatory cytokines, increased oxidative stress, and increased cortisol and insulin levels.
- Called this because it disrupts allostasis
What is allostasis?
- “Maintaining homeostasis through change”, works as nonlinear network.
- So too much/little of each mediator can be harmful and preturb whole network.
- This is because mediators reciprocally regulate each other.
- This means that too much or too little of each mediator can have harmful consequences by perturbing the entire network.
- This is because the mediators reciprocally regulate each other.
How does allostatic load come from sleep deprivation?
- It causes hypothalamic-pituitary-adrenal axis to be activated, and is done by sleep in 2 ways.
- Sleep deprivation activates HPA axis causing cortisol to be released. This leads to chronic stress related illnesses.
- Sleep deprivation also affects response to stress.
- Alterations in the HPA axis regulation may be caused by long sleep deprivation, but can also be induced by repeated occurrences of inadequate sleep.