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Evolutionary theory of sleep!
All animals appear to enter periods of quiescence
which might be described as sleep and all
mammals engage in something identifiable as
The fact that sleep evolved in all animals suggests that
the strong evolutionary pressures shaping this
behaviour must have been similar for them all.
A number of evolutionary explanations for
sleep have bee proposed. What they all have
in common, is the assumption that sleep
evolved because it served some purpose
useful to the fitness of the species.…read more

Slide 2

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Energy Conservation Theory.
· Webb (1982) claims that the function of sleep is
similar to that of hibernation.
· Animals which are adapted to diurnal activity
and eat during the daytime would be wasting
valuable energy at night when they would be
awake and unable to eat. Sleep then reduced
the amount of energy expended in an animal.
· Berger (1995) said that sleep helps to offset the
greater cost of being endothermic.…read more

Slide 3

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· Theory is supported by the observation that torpor and
hibernation, which undoubtedly serve energy
conservation functions, resemble SWS.
· Zeplin et al (2005) found that diet is strongly correlated
with sleep time, with herbivores sleeping the least and
carnivores sleeping the most. Herbivore diets are
relatively poor in nutrients so they must spend a lot of
their time sleeping whereas carnivores eat highly
nutritious food so eat less often . This gives them the
opportunity to sleep more. Omnivores eat both diets
and have sleep habits somewhere in between the two.…read more

Slide 4

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· The ecological habits and lifestyles of
some animals also point to an energy
conservation function.
· Lee and Martin (1990) describe the sleep
behaviour of the koala, a slow moving
animal which sleeps up to 19 hours a
day. For these animals, wasting energy
unnecessarily can be very costly indeed.
Similar conclusions can be drawn from
the behaviour of other animals with
poor diets, such as sloths and giant
pandas.…read more

Slide 5

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Supported by the extended amounts of sleep
observed in many newborn animals. Younger
animals have a high ratio of surface to body mass,
which makes energy conservation through sleep
particularly important. As body size increases, they
get less energy conservation benefit from sleep,
and so there is a decrease in sleep time.
· However... There is a major question
mark over whether sleep is actually an
effective way of conserving energy.…read more

Slide 6

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· Shapiro (1984) says that whilst there is a
reduction in body temperature during sleep,
sleep only saves us about 15% on energy and
even then this figure would depend on such
things as nutrition and lack of disturbance during
· It seems that whilst sleep might provide a way of
conserving energy in some animals, in others it
must be serving some other function.
· For example, Dolphins do not save any energy
while they sleep, as they must remain active at
all times, otherwise they'll drown.…read more

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