Transcript Theories of sleep - PsychologyResources-Y13

Animal sleep patterns
Sleep patterns vary
by species
less
4hrs
8hrs
16hrs
20hrs
more
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3hrs
Table 1. Representative
Total Sleep Requirements
for Various Species
Comparative studies of different species
generally state:
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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
WHY?
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
Evolutionary Theory
- we evolved to sleep so that we
would conserve energy when we
were least efficient
- predicts that species will sleep
different amounts depending on how
much they must look for food and
watch for predators
Evolutionary Explanation of Sleep
Examples:
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Energy
Conservation
Sleep uses less
energy than
wakefulness
Small animals with
high metabolic rates
need to conserve
their energy more
Mouse sleep
time: c. 12hrs
Foraging Requirements
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Herbivores spend
their time eating
plants, which are
relatively low in
nutrients
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They cannot afford to
spend too much time
sleeping
Horse sleep time: c. 2 hrs
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)
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Predator Avoidance
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If an animal is a
predator, it can
sleep longer than
prey species because
it has less predation
risk
Sleep should be
when an animal is
least vulnerable
Lions: c. 13.5 hrs
However: Waste of Time Theory
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Meddis (1975) sleep
helps animals stay
out of the way of
predators
Most animal should
sleep during darkness
Siegel (2008)
concurs. Sleep
enables both energy
conservation and
staying out of danger
Little brown bat: Awake when it
needs to be?
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?
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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 that 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.
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Waste of Time Theory – Meddis1975
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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?
Lets have a debate!!!
Restoration v Evolutionary
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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 can 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 begin this towards the end of our
revision class today