Functions of Sleep: Evolutionary Theory

Evolutionary Theory, AO1 and AO2

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AO1 - Energy Conservation

  • Mammals such as ourselves need to expend a lot of energy to maintain a constant body temperature
  • This is particularly problematic for small animals with high metabolic rates, e.g. mice
  • All activities use energy, and animals with high metabolic rates use even more energy
  • Sleep serves the purpose of providing a period of enforced inactivity (therefore using less energy), much as hibernation is a means of conserving energy
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AO1 - Foraging Requirements

  • If sleep is a necessity, the time spent sleeping may be constrained by food requirements
  • Herbivores such as cows and horses spend their time eating plants that are relatively poor in nutrients
  • As a result, they must spend a great amount of time eating, and consequently cannot afford to spend time sleeping
  • Carnivores such as lions and dogs eat food that is high in nutrients and so do not need to eat continuously
  • Therefore they can afford to spend time sleeping, and thereby conserve energy
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AO1 - Predator Avoidance

  • If an animal is a predator, it can sleep for longer
  • Prey species have a reduced sleep time as they must remain vigilant to avoid predators
  • Logically, to be safe they shouldn't sleep at all, but if sleep is a vital function then they are best off sleeping when least vulnerable
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AO1 - Waste of Time

  • Meddis: suggested that sleep helps animals to stay out of the way of predators during the parts of the day when they are most vulnerable
  • For most animals this means sleeping during the hours of darkness and it also means sleeping in places where they will be hidden
  • Sleep may simply ensure that animals stay still when they have nothing better to do with their time
  • Siegel: being awake is riskier than sleeping because an animal is more likely to be injured
  • The only possible explanation for sleep is that it enables both energy conservation and keeping the animal out of danger
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AO2 - Energy, Foraging or Protection?

  • Zepelin and Rechtshaffen: smaller animals, with higher metabolic rates, sleep more than larger animals (energy conservation = main reason for sleep)
  • HOWEVER there are many exceptions, e.g. sloths, which are very large yet sleep for 20 hours a day
  • Allison and Cicchetti: species who had a higher risk of predation slept less
  • HOWEVER there were exceptions, such as rabbits, who had a very high danger rating yet slept as much as moles, who had a low danger rating
  • Essentially, the research is very inconsistent
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AO2 - REM and NREM sleep

  • When considering energy conservation, there may be an important distinction between REM and NREM sleep
  • The energy consumption of the brain drops only in NREM sleep; during REM sleep the brain is still relatively active
  • It is potentially only NREM sleep that has evolved for energy conservation
  • Allison and Cicchetti also found that larger animals had less NREM sleep but not less REM sleep
  • HOWEVER Capellini et al found no correlation between NREM sleep and body size
  • Animals that are more 'primitive', such as most reptiles, only have NREM sleep
  • REM sleep appears to have evolved around 50 million years ago in birds and mammals
  • NREM sleep may have evolved first for energy conservation, whereas REM sleep may have evolved later to maintain brain activity
  • This is supported by the greater need for REM sleep in infants whose brains are developing
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AO2 - A Combined Approach

  • The evolutionary approach fails to address some of the key aspects of sleep, such as why we have such a strong drive for sleep when sleep-deprived
  • A combined approach recognises that some elements of sleep are for restoration whereas other aspects of sleep behaviour take up the function of unoccupied hours
  • Horne: suggested a distinction between core and optimal sleep
  • Core sleep is equivalent to SWS, and is the vital portion of sleep that an organism requires for essential body and brain processes
  • Optional sleep is dispensable, and has the function of occupying unproductive hours and, in the case of small mammals, conserving energy
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IDA - Mammalian Sleep

  • Capellini argued that previous research was flawed because the methods used to collect data were not standardised
  • She carefuly selected data from studies using only standardised procedures and focused only on land mammals (aquatic mammals have different sleep patterns)
  • They found a negative correlation between metabolic rate and sleep, which doesn't support the energy conservation hypothesis
  • However, this data supports the view that there is a trade-off between sleeping and foraging
  • Animals that sleep in exposed positions sleep less, but time spent sleeping is also reduced in species that sleep socially - safety in numbers
  • There is a strong phylogenetic signal (the behavioural similarities between species that are closely genetically related) for sleep among mammals
  • Mammals that are closely genetically related have more similar sleep patterns than would be expected by chance
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