Theories of Sleep

Theories of Sleep

Restoration Theory
1. 2 stages of sleep - slow-wave sleep (SWS) and REM sleep
  1. REM sleep enables brain recovery
  2. SWS enables body repair
2. Slow-wave sleep (SWS)
  1. Growth hormone is secreted during SWS
    1. growth hormone (GH) stimulates growth but also enables protein synthesis, cell growth, and tissue regeneration
    2. GH is secreted in pulses throughout the day
      1. but a significant amount is released at night and mainly during SWS
    3. Sassin (1969) - when sleep waking cycles are reversed by 12 hours, the release of GH is also reversed
      1. shows that GH release is controlled by neural mechanisms
  2. The immune system
    1. Kreugar (1985) - lack of SWS has also been associated with reduced functioning of the immune system
      1. immune system - consists of various protein molecules which are regenerated during cell growth and protein synthesis in SWS
3. REM sleep
  1. Brain growth
    1. the amount of REM sleep in any species is proportional to the immaturity of the offspring at birth
      1. EG. the platypus is immature at birth and has about 8 hours of REM sleep per day, whereas the dolphin, which can swim from birth, has almost no REM sleep
        suggests a relationship between neural development and REM sleep
  2. Neurotransmitters
    1. Siegel and Rogawki (1988) - REM sleep allows for a break in neurotransmitter release which permits neurons to regain their sensitivity and function properly
  3. REM sleep and memory
    1. research on dreams suggests a link between dreaming and memory
    2. REM may be important in the consolidation of procedural memory, whereas SWS is important for the consolidation of semantic memory and episodic memory
      Nota:
      - - procedural memory related to skills such as riding a bicycle - semantic memory related to knowledge and the meaning of things - episodic memory memory for events
Evaluation - Restoration Theory
1. Sleep deprivation
  1. Total sleep deprivation
    1. various case studies suggest that lack of sleep doesn't result in ling-term damage
      1. however, closely-monitored PP's who have been deprived from sleep for more than 72 hours display short episodes on microsleep while they're awake
        EEG recordings show that microsleep is the same as sleep
  2. Partial sleep deprivation
    1. may lead to 'rebound'
      1. EG. people awoken during REM sleep show up to 50% increases in REM activity
    2. same observed when SWS is prevented
    3. this rebound effect appears to be related to REM and SWS which suggests that there are the 2 kinds of sleep which are vital
2. Exercise and the need to sleep
  1. if the restoration theory is correct, physical exercise should lead to increased sleep in order to restore proteins and biochemicals
  2. Sharpio (1981) - marathon runners slept for about an hour more than usual on nights after the race
    1. however, Horne and Minard (1985) - PP's who were given numerous exhausting tasks didn't sleep more than usua. PP's went to sleep faster, but not longer
3. Comparative studies
  1. EEG studies of dolphins have found no evidence of REM sleep
    1. if REM sleep is vital to restoration, why don't dophins need it?
Evolutionary Theory
1. Energy conservation
  1. warm-blooded animals need to expend a lot of energy to maintain a constant body temperature
  2. all activities use energy and animals with high-metabolic rates use even more energy
  3. hibernation theory
    1. sleep serves the purpose of providing a period on enforced inactivity (using less energy)
2. Foraging requirements
  1. if sleep is a necessity for energy conservation, the time spent sleeping may be constrained by food requirements
  2. herbivores - spend their time eating plants, poor in nutrients, need to eat so can't 'afford' to sleep
  3. carnivores - eat food high in nutrients, can 'afford' to rest and conserve energy
3. Predator avoidance
  1. sleep is constrained by predation risk
  2. predators can 'afford' to sleep for longer
  3. prey species must remain vigilant so their sleep time is reduced
  4. to be safe they shouldn't sleep at all but if sleep is a vital function then they are best to sleep when least vulnerable
4. Waste of time
  1. Meddis (1975) - sleep may simply ensure that animals stay still when they have nothing better to do with their time
  2. Siegel (2008) - being awake is riskier than sleeping because an animal is more likely to be injured
    1. the only possible explanation for sleep is that it enables both energy conservation and keeping an individual out of danger
      1. EG. the little brown bat is awake for a few hours each day so it can eat the insects, and although it's high metabolic rate means it should eat more than sleep, it can get all it's food in a few hours so can just spend it's time sleeping
Evaluation - Evolutionary Theory
1. Energy
  1. Zepelin and Rechtschaffen (1974) - smaller animals with higher metabolic rates sleep more than larger animals
    1. supports the view that energy conservation might be the main reason for sleep
    2. however, sloths are large animals and sleep for 20 hours a day
2. Foraging and predation
  1. Capellini (2008) found a negative relationship between metabolic rate and sleep
    1. doesn't support the energy conservation hypothesis
  2. animals that sleep in exposed positions sleep less but sleep time was also reduced in species that sleep socially

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