AQA PSYA3 - Biological Rhythms - Evolutionary Explanation of Sleep

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Evolutionary Explanations

Tries to identify patterns of sleep across different species and key features determining these patterns e.g. body size, metabolic rate, brain size, predator or prey, safety of sleeping place.

Studies have found significant relationships...

(Alison & Cicchetti, 1976; Zepelin & Rechstaffen, 1975; Lesku et al, 2006)

  • Negative correlation between body mass and sleep time (small animals sleep more than big animals)
  • Brain mass is positively correlated with amount of REM sleep (those with larger, advanced brains have more REM)
  • Metabolic rate positively correlated with sleep time
  • Those sleeping in unsafe places have less sleep
  • Herbivores sleep less that carnivores (herbivores are often prey, carnivores are predators)


  • Correlation = association not cause & effect
    • small animals have higher metabolic rate - do they sleep more because of their size or metabolic rate?
  • Research often done in artificial environment such as zoos - may not accurately represent what happens in the wild.
    • Early studies on the sloth were carried out in zoos and found that they slept for 20 hours per day, longer than predicted for it's size and metabolic rate. Rattenborg et al (2008) found that in the wild they only sleep for 6 hours.
  • Not reductionist because it takes into account the whole animal & it's lifestyle - but cannot tell us the most important factor.

Meddis (1975)

Proposed that sleep is simply to keep animals safe at times when active behaviour is impossible.

  • Evidence for:
    • Most animals sleep at night when it…




Great evidence to use for meddis and webb, thanks so much 

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