Why do we sleep? – Evolutionary
Theory
BATs
AO1 – Understand and recall the ecological
theory of sleep
A02 – Analyse information and relate it to
psychological theories
- Use information from psychological
Good old
research to evaluate the theories
Approaches - Evolutionary , comparative
synoptic
points!!
Issues - use of non-human animals, Debates - Reductionism
Methodology – problems with the Phylogenic sleep project
Theories of sleep
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Hibernation theory (Webb, 1974)
Evolutionary approach – sleep is assumed to
confer a survival advantage
Sleep helps conserve energy by:

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Reducing energy spent on behavioural activity
Drop in body temp & metabolic rate reduces
energy lost as heat
psychlotron.org.uk

Hibernation theory
Main prediction is that an animal’s sleep
pattern will be determined by its energy
intake & expenditure pattern
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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)
Read and complete the worksheet – Ecological
Theories of Sleep
psychlotron.org.uk

Animal sleep patterns
Sleep patterns vary
by species
less
4hrs
8hrs
16hrs
20hrs
more
psychlotron.org.uk
3hrs
Evidence
Comparative studies of different species
generally support evolutionary view

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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)
Some notable exceptions e.g. rabbit (small,
herbivore) & human (much larger, omnivore)
both sleep about 8hrs
psychlotron.org.uk

Evidence
Sleep patterns are affected by energy
expenditure & availability

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Animals generally sleep more when weather is
cold and food is scarce (Berger & Phillips, 1995)
However, no direct correlation between physical
work done and sleep duration in humans (e.g.
Horne & Minard, 1985)
psychlotron.org.uk

Problems
Many evolutionary significant factors could
affect sleep patterns; theory doesn’t tell us
which are important

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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?
psychlotron.org.uk

Problems
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?
psychlotron.org.uk

Evaluation of Evolutionary
explanations
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A criticism of all evolutionary explanations is
that arguments about adaptiveness and
natural selection are made post hoc (‘after
the fact’ – after it happened).
We assume the sleep (and sexual selection!)
is adaptive because they are still around
today.
But they may just be neutral and not
selected out or linked to another adaptive
behaviour.
Evaluation of Evolutionary
explanations
The ecological (evolutionary) theory
of sleep is the same as Circadian
theory!
 Circadian rhythms evolved to induce
sleep at appropriate times of day – so
they are linked.

Waste of Time Theory – Meddis1975

1.
2.
3.
4.
5.
Read p 14 – Waste of time and Cutting
Edge and p15 – Unilateral sleep
What did Meddis propose?
Why did Siegel and Young (2008) support
Meddis.
What other explanation does their idea have
for why bats sleep so little?
What methodological issues are there with
the Phylogeny of Sleep project?
Why have some species adapted Unilateral
sleep?
A summary of the Hibernation/Ecological
Theory of Sleep
Compare the ideas of Webb (1982) and
Meddis (1975) – worksheet matching
activity
2. What evidence is there to support the
predictions outlined on the worksheet
(Ecological Theories of Sleep)?
3. What evidence is there that challenges
these ideas?
You can also use p 14-15 and the Worksheet
‘Theories of sleep – Hibernation theory’
1.
Lets have a debate!!!
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Which do you think is the most plausible
theory for why we sleep?
Jot down the pros and cons of each theory
Choose either the Restoration or Evolutionary
Theory.
Share all the good points you van think of
with others who have chosen the same
Theory
Be prepared to speak in a debate!!
Extension Activity
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Read Horne’s interview and page 15 – A
combined approach
Why might the Restoration theory and
Evolutionary theory be thought of as
Reductionist?
How does Horne’s theory address this
problem?
Homework
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
Discuss evolutionary explanations of the
function of sleep. (9+16 marks)
You can use the Restoration theory for A02
material as long as it is relevant.