How Animals Sleep

Fri, Sep 18, 2009


There are several theories about animal sleep:

lHibernation theory (Webb, 1974) lEvolutionary approach – sleep is assumed to confer a survival advantage lSleep helps conserve energy by: lReducing energy spent on behavioural activity lDrop in body temp & metabolic rate reduces energy lost as heat

Hibernation theory (Webb, 1974)

  • Main prediction is that an animal’s sleep pattern will be determined by its energy intake & expenditure pattern
  • Small animals will need more than large 
  • Diet (e.g. carnivore vs. herbivore) will affect sleep patterns
  • Sleep will increase when energy loss goes up (cold) or energy intake goes down (food scarcity)

Evolutionary approach 

  • Sleep is assumed to confer a survival advantage 
  • Sleep helps conserve energy by: 
    • Reducing energy spent on behavioural activity 
    • Drop in body temp & metabolic rate reduces energy lost as heat

Animal sleep patterns (vary by species)

                                                                                      3 hrs                        4 hrs              8 hrs                12 hrs               16 hrs              20 hrs

                                                                               Less                                                                                                                                                            More


  • Comparative studies of different species generally support evolutionary view
  • Smaller animals tend to sleep more than larger (e.g. giraffe 1hr vs. bat 20hrs)
  • Carnivores sleep more than herbivores (e.g. lion 16hrs vs. buffalo 3hrs)
  • lSome notable exceptions e.g. rabbit (small, herbivore) & human (much larger, omnivore) both sleep about 8hrs
  • Sleep patterns are affected by energy expenditure & availability 
  • Animals generally sleep more when weather is cold and food is scarce 
  • However, no direct correlation between physical work done and sleep duration in humans 


  • Many evolutionary significant factors could affect sleep patterns; theory doesn’t tell us which are important
  • Does a bat get so much sleep because it’s small or because it has few predators?
  • Why do animals with very different lifestyles have similar sleep patterns?
  • Some features of sleep cannot be explained easily by the hibernation theory: 
    • Why is sleep universal when in some species (e.g. dolphins) it would have been an advantage to get rid of it? 
    • Why is sleep deprivation apparently fatal?


If you can please post your answers below


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