Aqa A-level Psychology Pya4: Function Of Sleep
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Function Of Sleep Ecological Accounts
Observations of animals in their environment…
The Ecological Perspective •
Energy Conservation (Webb, 1982) Warm-blooded animals need to use up a lot of energy to maintain a constant body temp. A problem for small animals with a high metabolic rate (chemical processes taking place in the body). Animals with a high metabolic rate use much more energy. However… Sleep provides a period of enforced inactivity (using less energy) a means of conserving energy, much like hibernation (Webb, 1982) – Hibernation Theory. Sleep varies according to animal size and therefore metabolic rate. Amount of sleep is also changed by other factors: foraging (looking for food) and predator avoidance.
The Ecological Perspective •
Foraging (gathering food) Herbivores spend their time eating plants which are poor in nutrients therefore spend a lot of time eating = less time sleeping. Carnivores eat food that are high in nutrients therefore don’t need to eat continuously = afford to rest, conserving energy. •
Predator Avoidance (Meddis, 1975) Prey species must remain alert to avoid predator (reducing sleep). Meddis (1975) suggested that sleep help prey to stay out of harms way during parts of the day they are most vulnerable. For most animals this means sleeping during the hours of darkness however there are animals which have evolved to be nocturnal. According to Meddis, sleep may just ensure animals stay still when they have nothing better to do with their time.
A02 • -
-
-
-
Evolution of NREM and REM Sleep The brain uses a lot of energy, though energy consumption of the brain drops only in NREM sleep. During REM sleep the brain uses valuable energy since it is relatively active. Conclude that only NREM sleep has evolved for energy conservation. Supported by Allison & Cicchetti (1976) Animals that are more primitive (reptiles) have only NREM sleep. REM sleep appears to have evolved many years ago in birds and mammals. It may that REM sleep is adaptive only for animals with larger brains. They have a need to ‘exercise’ (use) their neural circuits and this apparently is done better in the absence of sensory inputs (i.e. when a animal is in a quiet, dark environment) NREM sleep may have evolved first for energy conversation and REM sleep evolved later to maintain brain activity. Suggesting that the energy view is likely to apply only to NREM sleep.
A02 • -
But does sleep actually conserve energy? An animal doesn’t conserve much energy than when resting (sleep reduces energy rates by 5-10%) Suggesting rest would be as adaptive as sleep for conserving energy. The risks associated with sleeping (i.e. loss of consciousness) would seem to outweigh borderline advantage of ‘energy conservation’. Basically, this means that the ‘energy conservation’ explanation (Webb) is not sufficient to explain the function of sleep.
-
In addition the unilateral (one sided/one eye) sleeping habits of marine mammals/birds indicate that sleep function can be satisfied while partially active. Suggesting that energy conservation can’t be a key adaptive feature of sleep.
A02 • -
Does sleep assist predator avoidance? Some animals (i.e. ducks) solve the alertness problem through unilateral sleep, so why hasn’t this evolved in more prey species? We could ask why animals so not simply rest rather losing consciousness.
-
Answer: It may be that loss of consciousness ensures greater stillness than rest alone. Unilateral sleep may not be more common because the evolutionary advantages may not be great enough.
Question Time 1.
What is described as the Hibernation Theory (Webb, 1982)?
3.
What factors also modify amount of sleep?
5.
What does Meddis (1975) suggest about predator avoidance?
7.
What type of sleep has evolved to converse energy?
9.
What is meant by unilateral sleep?
11.
Why hasn’t unilateral sleep evolved in more prey species?
Research Studies • Sleep length and energy conservation Zepelin & Rechtschaffen (1974) – studied relationship between sleep length and metabolic rate rate across 53 species (mammals). Found a negative correlation between body size and total sleep time, indicating that animals with higher metabolic rates sleep for longer. Small animals spend more hours a day asleep while larger animals sleep relatively little (i.e. elephants sleep for about 4hrs a day) • Sleep length and predator avoidance Allison & Cicchetti (1975) – studied relationship between sleep in animals and the degree of danger experienced (in a lab) and found in 39 species amount of sleep in an animal negatively correlated with the amount of danger experienced. Basically, the greater the environmental danger, the less time an animal spent sleeping per day.
Research Studies •
Unilateral Sleep Dolphins need to swim to the surface to take a breath. If they fell into a deep sleep, they would drown. Evolution solution: only one half of the dolphins brain is in NREM sleep at a time, the other half staying awake. Two hemispheres of the brain swap every 2-3 hrs – Mukhametov, 1987 Birds also display unilateral sleep. Rattenborg et al (1999) – showed birds on the end of a row who were watching out for predators were more likely to sleep with one eye opened than the birds in the middle. Recordings of brain activity in sleeping mallard ducks showed the brain hemisphere corresponding to the open eye is awaken while the one for the closed eye is in deep sleep.
Research Studies - Evaluation • -
Sleep length and energy conservation The relationship that animal size is related to time spent sleeping isn’t true in all species. Sloth's are very large and slow so they shouldn’t need much sleep however they sleep 20hrs a day.
• -
Sleep length and predator avoidance Animals don’t always spend less time sleeping when environmental dangers are high. Rabbits were found to sleep as much as moles, but rabbits had very high danger ratings and moles had very low ratings
• -
Researchers are finding things out all the time Behaviour of animals has yet to be accurately recorded. It was only recently that platypus sleep habits were carefully recorded and found it does have REM sleep (being a rather primitive mammal) – Siegel et al. 1998
Question Time 1.
What did Zepelin & Rechtschaffen find?
3.
Who found that the greater the environmental danger, the less time an animal spent sleeping per day?
5.
What was the evolution solution for the dolphin problem? (if they fell into deep sleep = drown)
7.
Give a example why the relationship between size and time spent sleeping isn’t always true?
9.
What did Siegel et al. 1998 find?
Observations of animals in their environment…
The Ecological Perspective •
Energy Conservation (Webb, 1982) Warm-blooded animals need to use up a lot of energy to maintain a constant body temp. A problem for small animals with a high metabolic rate (chemical processes taking place in the body). Animals with a high metabolic rate use much more energy. However… Sleep provides a period of enforced inactivity (using less energy) a means of conserving energy, much like hibernation (Webb, 1982) – Hibernation Theory. Sleep varies according to animal size and therefore metabolic rate. Amount of sleep is also changed by other factors: foraging (looking for food) and predator avoidance.
The Ecological Perspective •
Foraging (gathering food) Herbivores spend their time eating plants which are poor in nutrients therefore spend a lot of time eating = less time sleeping. Carnivores eat food that are high in nutrients therefore don’t need to eat continuously = afford to rest, conserving energy. •
Predator Avoidance (Meddis, 1975) Prey species must remain alert to avoid predator (reducing sleep). Meddis (1975) suggested that sleep help prey to stay out of harms way during parts of the day they are most vulnerable. For most animals this means sleeping during the hours of darkness however there are animals which have evolved to be nocturnal. According to Meddis, sleep may just ensure animals stay still when they have nothing better to do with their time.
A02 • -
-
-
-
Evolution of NREM and REM Sleep The brain uses a lot of energy, though energy consumption of the brain drops only in NREM sleep. During REM sleep the brain uses valuable energy since it is relatively active. Conclude that only NREM sleep has evolved for energy conservation. Supported by Allison & Cicchetti (1976) Animals that are more primitive (reptiles) have only NREM sleep. REM sleep appears to have evolved many years ago in birds and mammals. It may that REM sleep is adaptive only for animals with larger brains. They have a need to ‘exercise’ (use) their neural circuits and this apparently is done better in the absence of sensory inputs (i.e. when a animal is in a quiet, dark environment) NREM sleep may have evolved first for energy conversation and REM sleep evolved later to maintain brain activity. Suggesting that the energy view is likely to apply only to NREM sleep.
A02 • -
But does sleep actually conserve energy? An animal doesn’t conserve much energy than when resting (sleep reduces energy rates by 5-10%) Suggesting rest would be as adaptive as sleep for conserving energy. The risks associated with sleeping (i.e. loss of consciousness) would seem to outweigh borderline advantage of ‘energy conservation’. Basically, this means that the ‘energy conservation’ explanation (Webb) is not sufficient to explain the function of sleep.
-
In addition the unilateral (one sided/one eye) sleeping habits of marine mammals/birds indicate that sleep function can be satisfied while partially active. Suggesting that energy conservation can’t be a key adaptive feature of sleep.
A02 • -
Does sleep assist predator avoidance? Some animals (i.e. ducks) solve the alertness problem through unilateral sleep, so why hasn’t this evolved in more prey species? We could ask why animals so not simply rest rather losing consciousness.
-
Answer: It may be that loss of consciousness ensures greater stillness than rest alone. Unilateral sleep may not be more common because the evolutionary advantages may not be great enough.
Question Time 1.
What is described as the Hibernation Theory (Webb, 1982)?
3.
What factors also modify amount of sleep?
5.
What does Meddis (1975) suggest about predator avoidance?
7.
What type of sleep has evolved to converse energy?
9.
What is meant by unilateral sleep?
11.
Why hasn’t unilateral sleep evolved in more prey species?
Research Studies • Sleep length and energy conservation Zepelin & Rechtschaffen (1974) – studied relationship between sleep length and metabolic rate rate across 53 species (mammals). Found a negative correlation between body size and total sleep time, indicating that animals with higher metabolic rates sleep for longer. Small animals spend more hours a day asleep while larger animals sleep relatively little (i.e. elephants sleep for about 4hrs a day) • Sleep length and predator avoidance Allison & Cicchetti (1975) – studied relationship between sleep in animals and the degree of danger experienced (in a lab) and found in 39 species amount of sleep in an animal negatively correlated with the amount of danger experienced. Basically, the greater the environmental danger, the less time an animal spent sleeping per day.
Research Studies •
Unilateral Sleep Dolphins need to swim to the surface to take a breath. If they fell into a deep sleep, they would drown. Evolution solution: only one half of the dolphins brain is in NREM sleep at a time, the other half staying awake. Two hemispheres of the brain swap every 2-3 hrs – Mukhametov, 1987 Birds also display unilateral sleep. Rattenborg et al (1999) – showed birds on the end of a row who were watching out for predators were more likely to sleep with one eye opened than the birds in the middle. Recordings of brain activity in sleeping mallard ducks showed the brain hemisphere corresponding to the open eye is awaken while the one for the closed eye is in deep sleep.
Research Studies - Evaluation • -
Sleep length and energy conservation The relationship that animal size is related to time spent sleeping isn’t true in all species. Sloth's are very large and slow so they shouldn’t need much sleep however they sleep 20hrs a day.
• -
Sleep length and predator avoidance Animals don’t always spend less time sleeping when environmental dangers are high. Rabbits were found to sleep as much as moles, but rabbits had very high danger ratings and moles had very low ratings
• -
Researchers are finding things out all the time Behaviour of animals has yet to be accurately recorded. It was only recently that platypus sleep habits were carefully recorded and found it does have REM sleep (being a rather primitive mammal) – Siegel et al. 1998
Question Time 1.
What did Zepelin & Rechtschaffen find?
3.
Who found that the greater the environmental danger, the less time an animal spent sleeping per day?
5.
What was the evolution solution for the dolphin problem? (if they fell into deep sleep = drown)
7.
Give a example why the relationship between size and time spent sleeping isn’t always true?
9.
What did Siegel et al. 1998 find?
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