Functions of Sleep: Evolutionary Theory

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  • Functions of Sleep: Evolutionary Theory
    • Energy Conservation
      • warm blooded animals use a lot of energy to maintain a constant body temperature.
      • Problem for small animals as they will need to sleep more to maintain the energy they use for keeping their body temp constant.
      • Sleep provides animals with a period of enforced inactivity where the animals won't use any energy.
        • This is the hibernation theory of sleep.
    • Foraging Requirements
      • Herbivores eat plants that are low in nutrients so must spend a lot of time eating and less time sleeping.
      • Carnivores eat food high in nutrients so don't need to eat as much so can sleep most of the time and conserve energy.
    • Predator Avoidance
      • Sleep is constrained by predation risk.
      • Predators can sleep for longer as there is practically no risk.
      • Sleep time is reduced for prey as they have to stay alert because of predators.
    • Waste of time
      • Meddis - sleep helps animals stay out of the way of predators when they are most vulnerable.
      • Sleeping when they have nothing else better to do.
      • Siegel - when an animal sleeps they are more likely to be injured as they are not alert.
      • Staying awake when you need to - to get food or get away from predators.
    • If we compare sleep habits across different species, we can see the costs and benefits of sleep.
      • Zeplin and Rechtschaffen found that smaller animals sleep more than larger animals because of their higher metabolic rates. However, sloths are large animals but sleep for 20 hours a day.
        • The fact that smaller animals sleep more than larger animals supports the view that energy conservation may be the main reason for sleep. However, the sloth is the exception.
    • There may be an important distinction between NREM and REM sleep.
      • As we know, the brain is active in REM sleep but in NREM sleep the energy consumption of the brain drops. Allison and Ciccheti found larger animals had less NREM sleep but not less REM sleep.
        • This supports the view that NREM sleep has evolved for energy conservation purposes.

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