Ecological Function of sleep
Sleep has an important adaptive value. Sleep conserves energy at times when food is scarce and enables animals to avoid predators.
Explanation 1: ENERGY CONSERVATION (WEBB, 1982)
A01: Energy Costs
- Maintaining body temperature and having a high metablolic rate (chemical processes in body) have a physiological cost; foraging and escaping from predators also have an energy cost.
A02: Supported by...
ALLISON and CICCHETTI (1976), who found that larger animals experience less NREM sleep but not less REM sleep, showing that only NREM sleep is important in energy conservation.
A01: Energy Conserved:
- Sleep is enforced inactivity (like hibernation)
A02: It may be that...
- NREM sleep evolved first (reptiles don't have REM sleep) for energy conservation
- REM sleep evolved later in animals with larger brains - to exercise neural circuits.
A01: Negative Correlation
- The amount of sleep is negatively correlated with body size. This relationship is further modified by foraging needs and predator danger.
A02: However... energy conservation in sleep is minimal (5-10%) and the risks are large. Energy can be conserved by partial inactivity (e.g. unilateral sleep)
Explanation 2: FORAGING REQUIREMENTS
A01: Animals with higher metabolic rates
- Such animals sleep for longer (ZEPELIN and RECHTSCHAFFEN, 1974)
A02: However... despite this finding, some species (e.g. the sloth) contradict this trend
A01: Sleep duration
- This is influence by foraging requirements. Herbivores eat food low in nutrients and sleep little. Carnivores eat food rich in nutrients so sleep longer.
A02: This account offers an explanation for why... there is so much variety in sleep patterns among species - they must adapt to the different pressures of their environments.
Explanation 3: Predator avoidance (MEDDIS, 1975)
A01: Negative correlation: The amount of sleep taken by an animal correlates negatively with the amount of danger typically experienced (ALLISON and CICCHETTI, 1976)A02: Disadvantages of sleep: Sleep is a costly behaviour as while asleep, animals cannot be vigilant against predators, forage or protect their young.
A01: Prey species sleep less to be vigilant
- But if they have to sleep it is best to do so when they are least vulnerable (night)
- It is best to stay still (i.e. sleep) when there is nothing better t do.
A02: However, it is possible that the greater 'stillness' of sleep renders prey species safer from predators than when simply resting.
A01: Unilateral sleep: Dolphins and some birds display unilateral sleep to maintain predator vigilance whilst conserving energy.
A02: However, BOEREMA et al. (2003) found that sleep-deprived chickens slept with both halves of their brain at the cost of reduced alertness.
FUNCTION OF SLEEP: ECOLOGICAL (STUDIES)
Study 1: Sleep length and energy conservation:
There is a negative correlation betewen the body size of a species and the amount of time spent sleeping. Small animals (e.g. brown bats) spend more hours asleep than larger animals (e.g. elephants) (ZEPELIN and RECHTSCHAFFEN, 1974)
A02: This supporst the claim that... animals with higher metabolic rates need to conserve energy by sleeping for longer periods.
A02: However... ALLISON and CICCHETTI (1976) found that larger animals experience less NREM sleep but not less REM sleep, showing that only NREM sleep is important for energy conservation
Study 2: Endothermy and NREM Sleep
Research has provided evidence for the universal presence of NREM sleep in endothermic (warm-blooded) mammals and birds and its absence in reptiles and other ectothermic (cold blooded) species
- NREM sleep is seen to have evolved as a way of conserving the energy needed to maintaining endothermy.
KAVANAU (2004) claims that the loss of muscle tone associated with NREM sleep would have been insufficient to prevent muscle contractions during sleep.
Study 3: Sleep length and predator avoidance
ALLISON and CICCHETTI (1976) found that the more danger typically faced by a species, the less it slept.
A02: This supports the claims that...
- Evolution of sleep is associated with the need to remain safe from predators.
- Large amounts of REM sleep are disadvantageous to prey species.
However... data are correlational, i.e. do not demonstrate a causal relationship.
Also... the inverse relationship between sleep and predator avoidance does not hold true to all animals, e.g. rabbits (high danger) sleep the same as moles (low danger rating)
Study 4: Unilateral sleep
- Birds watching for predators keep one eye open and corresponding hemisphere active. (RATTENBERG et al,1999)
- MUKHAMETOV (1984) found that bottlenose dolphins 'switch off' one hemisphere at a time. During sleep they are still able to come up to the surface to breathe. A02: This shows that... Marine mammals are able to gain the benefits of sleep while at the same time avoiding dangers in their environment.
However... BOEREMA et al. (2003) found that other birds (e.g. chickens) abandon unilateral sleep when sleep-deprived at the cost of reduced alertness.
Study 5: REM sleep and developmental complexity
- ZEPELIN (1989) has shown that altricial animals (unable to care for themselves) have much larger amounts of REM sleep at birth than precocial mammals, with immaturity at birth the single best predictor of REM sleep time throughout life. A02: This suggests that... there us a link between the need for REM sleep and greater developmental complexity.
Function of Sleep: RESTORATION (EXPLANATIONS)
A01: REM sleep is important for brain growth and repair whereas SWS is important for bodily restoration.A02: Research support: Case studies of total sleep deprivation (Peter Tripp, Randy Gardner) have provided evidence for the body's resilience to deprivation of sleep.A02: However... these case studies have three main limitations: Unique characteristics of the participants, contradictory findings and lack of scientific control.
EXPLANATION 1: REM SLEEP AND BRAIN RECOVERY (OSWALD, 1969)
A01: Drug overdose
Patients recovering from drug overdoses and CNS injuries show significant increases in REM sleep during their recovery period.
A02: However... research has found little evidence that intense physical exercise does anything other than make people fall asleep faster.
Large amount of REM sleep in babies reflects rapid brain growth.
A01: Replenishment of neurotransmitters
During REM sleep, neurons that have been active during the day cease firing but continue to synthesis new neurotransmitters for the next day.
A02: A problem with this explanation is that... REM sleep involve considerable neural activity, which uses up neurotransmitters. Their replenishment cannot, therefore, be the sole function of REM sleep.
A02: A further problem for this explanation is that... symptoms of severe depression are reduced in some people when deprived of REM sleep; also there is some contradictory evidence that complete REM deprivation has no significant ill effects.
EXPLANATION 2: SWS and BODILY RESTORATION (OSWALD,
A01: Hormonal Activity-The increase in the body's hormonal activities during SWS suggests a restoration process in the body.
- A02: Supported by the finding that SWS deprivation in fibrositis sufferers leads to non-restorative sleep patterns.
- SWS is associated with the secretion of growth hormones, important for the synthesis of proteins, which need to be constantly restored.
- A02: However, HORNE (1988) highlighted the fact that amino acids are only freely available for 5 hours after a meal, making proteins synthesis during SWS unlikely.
A01: Increase in NREM sleep after deprivation
- BERGER and OSWALD (1962) found a marked increase in NREM on the first night after sleep deprivation while REM remained the same.
- A02: However... some research has found no differences in the effects of partial sleep deprivation (REM or SWS) or total sleep deprivation (REM and SWS)
Explanation 3: CORE SLEEP (HORNE, 1988)
A01: CORE SLEEP- Core sleep = REM + SWS, which is responsible for normal brain functioning (HORNE, 1988)
- A02: This Explanation is supported by... HORNE (1988), who reviewed 50 studies where people had been deprived of sleep - very few reported that this interfered with the ability to perform physical exercise.
- A02: Also... runners slept for only an hour more after physical exertion (SHAPIRO et al., 1981)
A01: Lighter stages of NREM: These are not essential and are therefore optional sleep
A01: Bodily Restoration: This takes place during periods of relaxed wakefulness, while core sleep provides for restoration of brain systems.
A02: The view that sleep is necessary for bodily restoration is challenged by... the finding that participants given exhausting tasks went to sleep faster but not for longer (HORME AND MINARD 1985)
FUNCTION OF SLEEP: RESTORATION (STUDIES)
Sleep deprivation studies are used to test the assumption that sleep has an important restorative function.
STUDIES 1: TOTAL SLEEP DEPRIVATION
- Peter Tripp - After 3 days he becamse of abusive, after 5 days he hallucinated and became paranoid, after 8 days his brain waves resembled those of a sleeping state. After 24 hours' sleep he recovered.
- Randy Gardner - After 11 days no abnormal symptoms were present.
- There are 3 main limitations of these case studies: the unique characteristics of each participant, contradictory findings and lack of scientif control.
- However... it is impossible to prevent microsleep in these studies, so participants may not be completely sleep-deprived.
- In general... case studies show no significant effects of sleep deprivation.
STUDIES 2: REM SLEEP DEPRIVATION
REM sleep deprivation increases the tendency to enter REM sleep earlier and increase the proportion of REM sleep on subsequent nights (REM rebound) (EMPSON, 2002)
A02: However... symptoms of severe depression are reduced in some people when deprived of REM sleep.
A02: Some contradictory evidence... exists that complete REM deprivation has no significant ill effects e.g. one brain-injured patient who led a normal life without REM sleep (LAVIE et al., 1984)
STUDIES 3: SWS SLEEP DEPRIVATION -
MOLDOFSKY et al. 1975 found that:
- Fibrositis patients experience tiredness and EEG patterns in SWS are faster (therefore SWS is not restorative)
- Volunteers deprived of SWS sleep experienced symptoms of fibrositis.
A02: However, some research has found no differences in the effects of partial sleep deprivation (REM or SWS) or total sleep deprivation (HORNE, 1988)
A02: Also... SWS effects are unlikely - proteins synthesis uses amino acids which are not available 5 hours after a mean (HORNE, 1988)
Studies 4: HORNE'S EXPLANATION
- BERGER AND WALKER (1972) found a large increase in SWS at the expense of the later stages on the first night after deprivation. There was an increase in REM on subsequent nights.
- A02: This supports the view that... stage 4 and REM sleep constitute 'core' sleep, as participants were almost completely recovered following deprivation, and that lighter stages have no restorative function.
- STERNE and MORGANE (1974) found no evidence of REM rebound following sleep deprivation for patients on anti-depressants.
- A02: This can be explained because... anti-depressant drugs increase levels of available dopamine and serotonin, so there is no need for an REM rebound.