Animal sleep patterns

less more

4hrs 20hrs16hrs3hrs 8hrs

Sleep patterns vary by species

psyc

hlot

ron.

org.

uk

Table 1. Representative Total Sleep Requirements

for Various Species

Comparative studies of different species generally state: 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?

psyc

hlot

ron.

org.

uk

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:

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

Herbivores spend their time eating plants, which are relatively low in nutrients

They cannot afford to spend too much time sleeping

Horse sleep time: c. 2 hrs

Evidence

Sleep patterns are affected by energy expenditure & availability 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)

psyc

hlot

ron.

org.

uk

Predator Avoidance

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

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 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?

psyc

hlot

ron.

org.

uk

Evaluation of Evolutionary explanations

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.

Waste of Time Theory – Meddis1975

Read p 14 – Waste of time and Cutting Edge and p15 – Unilateral sleep

1. What did Meddis propose?2. Why did Siegel and Young (2008)

support Meddis.3. What other explanation does their idea

have for why bats sleep so little?4. What methodological issues are there

with the Phylogeny of Sleep project?5. Why have some species adapted

Unilateral sleep?

Lets have a debate!!!Restoration v Evolutionary

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

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

Discuss evolutionary explanations of the function of sleep. (9+16 marks)

You can begin this towards the end of our revision class today