Function of sleep

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  • Function of sleep
    • Evolutionary theories: sleep provides a survival advantage
      • Energy Conservation theory: Webb
        • for animals where food is scarce sleep is the most beneficial way to conserve energy between meals; it is not guaranteed when it will be
          • however herbivorous tend to sleep less as they have to  spend longer eating because their non-meat diet is less nutritious so to conserve their energy they spend more time eating
        • there is clear evidence for this theory in findings that the basal metabolic rate (BMR)(the rate at which the body burns up energy is positively correlated with sleep time (Berger and Phillips) - the higher the BMR the longer the animal sleeps as they burn energy faster so sleep more to conserve it
          • small animals have higher BMR so sleep more
          • however only N-REM can contribute to energy conservation as the energy consumption in REM is only slightly less than waking behaviour
          • Savage and West- suggested that brain metabolic rate was more important than BMR when determining sleep time- they studied 96 species and sleep time was positively correlated with BMR- but there was a stronger positive correlation with brain metabolic rate
        • Lesku- herbivores who have less nutritious diets spent less time in REM where energy conservation is similar to waking- supports
      • Threat simulation theory: Revonsuo
        • we sleep so that in our dreams we can 'practice' how we would respond to potential threats and therefore be more prepared in real life when faced with them and have a better chance of surviving the situation
          • williamson et al- stressfull events tend to increase length and frequency of REM (dream sleep)
            • Revonsuo- dream content is usually bias towards real life threatening events and Brenneis- real life threatening event often feature in dreams
      • Predation theory: Meddis
        • sleep behaviour provides a survival advantage as being unconscious and thus not moving or attracting predator attention
          • however surely being awake all of the time would provide more of a survival advantage so this is questionable and there must be other influences
          • animals with poor night vision are vulnerable to nocturnal animals in the night so they develop strategies to prevent attack e.g. remaining alert or hiding e.g. warthogs who build burrows sleep for longer
          • Allison and Ciccetti- found a correlation between risk of predation and amount of sleep- however this only applies to some animals as most prey animals are herbivores and Lesku found that herbivores spend less time sleeping.
            • Lesku- found that the more dangerous the sleep site the less REM- the body loses muscle tone during REM so escape is more difficult
          • Snyder's sentinal hypothesis- supports the theory and states animals are easily aroused from REM and quickly alert in case escape is needed
            • Cote- animals are very sensitive to external stimuli when asleep
    • any explanation must explain why newborn babies sleep for 16 hours a day 50% of which is REM but gradually reduce until they reach the adult sleep pattern of 6-8 hours sleep
    • Restoration theories state that sleep is necessary for brain and bodily repair
      • Oswald's restoration theory was developed when he noticed that people suffering from severe brain injury spent more time in REM and that our skin cells grow faster in our sleep
        • he stated that N-REM sleep is needed for body repair; as seen in the increase in growth hormone during sleep: and REM is needed for brain restoration
          • this explains why newborns spend a third of their day in REM- it is a time of massive brain growth- and it explains why in REM brain energy consumption is close to waking
          • Oswold predicted that burning more resources during the day leads to longer sleep time and that sleep deprivation should lead to and effect on brain function
            • evidence can be found in physical exercise studies
              • Shapiro found that ultramarathon runners spend longer asleep and more time in REM for up to 2 days after a marathon with more deep N-REM- where the growth hormone is released most - supports Oswald
              • however Horne and Minard found that physical exercise leads to people going to sleep faster but not for longer
            • Total sleep deprivation studies can also offer some support for Oswald's theory
              • Tripp- stayed awake for 201 hours and suffered hallucinations and paranoia 9supporting O's first prediction of brain disturbance) but slept for only 24 hours and was fine when according to O- he should have recovered all of his sleep lost in that 8 days
              • Randy Gardener- stayed awake for 264 hours and suffered blurred vision and disorganized speech and paranoia- slept for only 15 hours on his first recovery night and recovered only a 1/4 of the sleep he lost overall in his 11 days awake
                • these are unreliable as they were not controlled and may have taken micro-naps to keep them going: the invention of the sleep laboratory makes research like this more scientific
              • these are case studies but can conclude that TSD effects mental function: after TSD only a small amount of sleep is recovered mainly REM and deep N-REM
                • animal studies have found that sleep deprived rats can survive only 19 days before they die- however the procedure that keeps them awake is so severe it may be the cause of the death
            • however according to this paralyzed people shpuld need less sleep to restore their bodies according to restoration theory however Adey et al- found that they only experience a small reduction in deep sleep
      • Horne's restoration theory
        • Horne and Pettitt- wanted to see if incentives would help people overcome sleep deprivation
          • group A were sleep deprived for 72 hours and given no incentive; group B were sleep deprived for 72 hours and given a financial incentive; group C were the control group and not sleep deprived
            • tested on an auditory vigilance task- Group B performed the same as the control group after 1 night of sleep deprivation, and still performed better than group A after 2 nights
              • all participants were fully recovered after 8 hours sleep
          • they concluded that sleep affects motivation more than anything
        • he proposed that REM and deep N-REM were essential for normal brain function and known as 'core sleep'
          • light N-REM appears to have no function apart from keeping the individual inconspicuous- known as 'optional sleep'
        • can't explain why theres a surge in growth hormone as we enter N-REM as there is no amino acids at that point to absorb the hormone so the surge is not for growth
      • Stern and Morgane argue that the function of REM is linked to the synthesis of neurotransmitters used up in daytime activities. antidepressants increase these chemicals which explains why People on them can go on with less REM

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