Theories of Sleep Deprivation

Theories of SleepA theory of sleep is an attempt to explain whyanimals sleep. A good theory of sleep must fulfila number of criteria including:• It should provide a plausible explanation asto why sleep is found in such a variety ofdifferent animals.• It should explain the findings of researchstudies into sleep deprivation.• It should be able to explain the differencesthat exist in sleep patterns between differentspecies.There are two general approaches taken by theories ofsleep. On the one hand, restoration theories suggestthat sleep exists in order to repair and restore the body.On the other hand, there are evolutionary theories thatrelate sleep to the ability to survive in a hostileenvironment.Restoration Theory (Oswald, 1966)According to Oswald (1966), the function of sleep isto restore the body during periods of inactivity so thatadequate biological functioning is ensured. Thetissues of the brain and body are repaired and thechemicals needed for proper functioning arereplenished. Specifically, Oswald claims that:• NREM sleep restores biological processesthat have deteriorate during the day;• REM sleep replenishes and renews brainprocesses through the process of proteinsynthesis.Evidence for Restoration Theorywww.psychlotron.org.ukSleep Deprivation Studies(see previous handout)Many animal studies of sleep deprivation support theideas of the restoration theory because they show thatbiological functioning deteriorates if an animal isdeprived of sleep. However, the results of humanstudies are not as clear cut and do not seem to supportthe idea that biological functioning is upset if a humanis deprived of sleep.Changes in Sleep Patterns over the LifespanBetter support for Oswald comes from studies of sleeppatterns as a child grows older. Babies sleep far morethan older children and adults, and they spend muchmore of their sleep time in REM (see fig. 1).Fig. 1 – Changes in sleep pattern over the lifespan24 hour periodAWAKEREMNREM5yrsBabies spend about 18 hours in every 24 asleep, andabout half of this in REM. By the age of five, this hasfallen to about 8 hours in every 24, only a quarter inREM, which remains basically the same for the rest oftheir life. Since the first five years are very importantfor brain development, where a great deal of proteinsynthesis is necessary for cell growth andmanufacture, it makes sense that a younger childwould require far more REM sleep than an adult.Sleep Patterns Following Brain InsultIf REM sleep is necessary for proper brainfunctioning, then we would expect increases in REMsleep in people whose brains had been adverselyaffected by something. This is exactly what is found.People recovering from brain injury, electroconvulsivetherapy and drug withdrawal tned to spendmore time in REM during their recovery period.Sleep Patterns and IllnessIf sleep is required for immune functioning, then wemight expect total sleep time to increase duringperiods of illness. Again, this is observed in humansand other animals.Hormonal Fluctuations During SleepThere is evidence that secretion of growth hormone bythe pituitary gland increases during stage 4 sleep.Growth hormone is necessary for normal functioningof the body, and a deficiency can result in musculardisorders. People with a condition called fibrositis(which results in inflammation of the muscles) tend tohave abnormal sleep patterns, with very littleuninterrupted stage 4 sleep. Gross (1996) points outthat prolonged deprivation of stage 4 sleep inducesfibrositis symptoms in healthy volunteers.EvaluationAll of this evidence is consistent with the idea thatsleep serves the purposes of restoring and replenishingbiological processes and functions. However, thereare a couple of objections that are worth noting:• Studies of sleep deprivation in humans donot so far indicate that it results in seriousbiological malfunctions.• Cell repair has been shown to carry on for24 hours a day and is therefore not confinedto sleep periods (although it does increaseduring sleep periods).Contributed by Aidan Sammons

• During REM sleep the brain is very active,and this would actually prevent proteinsynthesis from taking place.The final objection does not mean that sleep is not forrestoration, but makes it unlikely that REM sleep isspecifically for the restoration of the brain, as Oswaldclaims.Hibernation TheoryHibernation theory (Webb, 1974) is one of a numberof evolutionary theories of sleep. Evolutionarytheories in general suggest that sleep evolved becauseit allows an organism a greater chance of survival in ahostile environment.Specifically, Webb suggests that sleep has evolvedbecause it forces the animal to conserve energy. Ingeneral, animals that conserve energy are more likelyto survive than animals that do not. Therefore, thecharacteristic that helps them conserve energy is morelikely to be passed on to the next generation. Sleephelps an animal conserve energy in a number of ways.For example:• During sleep, behavioural activity stops,meaning less energy is spent on movement.• Body temperature and metabolic rate slowdown, again, saving energy.According to Webb, an animal’s sleep patterns will bedetermined by the rate at which it loses heat, what iteats and the availability of food. By comparing thesleep patterns of different species at different times, itis possible to see whether these factors appear to havean effect.Evidence for Hibernation TheoryRate of Heat LossThe smaller an animal is, the faster it loses heat. Sinceheat loss is a major waste of energy, we might expectthat smaller animals will sleep more than larger ones.This seems to be true. Bats, voles and other smallmammals sleep for as many as 20 hours in every 24,whilst giraffes sleep for as little as 1 hour.DietAn animal’s diet determines its rate of energy intakeand this in turn determines how long it must spendeach day searching for food. Carnivores eat large,nutritious meals and therefore don’t have to spendmuch of each day eating. Lions, for example, sleepfor as many as 18 hours a day, possible more if theyhave just made a large kill. One the other hand, horseseat grass, which contains very little nutrition. For thisreason, they must eat a massive amount just to stayalive. This takes a great deal of time and they sleepfor only 3 hours in every 24.Food AvailabilityHibernation theory would predict that sleep timeshould increase when food is scarce, because theanimal has a greater need to conserve energy.According to Berger & Phillips (1995) this is indeedthe case.EvaluationThe main problem with hibernation theory is that thereare a whole host of evolutionary factors that couldpotentially affect sleep patterns. For example, preyanimals are at greater risk of attack than predators.Therefore, we might expect buffalo (4 hours per day)to sleep less than lions, as a buffalo must be on thelookout for predators, whereas a lion does not. Thereare in fact two problems here:• Which factors are actually important? Doesa bat get lots of sleep because it is small, orbecause it has few predators?• Similarly, why do animals with verydifferent lifestyles get similar amounts ofsleep? For example, both humans andrabbits get about 8 hours in every 24 buthave very different body sizes, diets andrelationships with predators.Ultimately, there is no variation is sleep pattern thatcannot be ‘explained’ by using evolutionary ideas.But at the same time, there is no way of decidingwhich evolutionary factors are important. This meansthat Webb’s evolutionary theory of sleep isunfalsifiable. A theory that is unfalsifiable is not ascientific theory.Another major problem with the hibernation theory isthat it does not explain why sleep is universal. Surelyit would be possible to find evidence of animals thatdo not sleep, because there is no evolutionaryadvantage in doing so?• However, all mammals sleep. Dolphinssleep with half of the brain at a time – surelyif the only function of sleep was energyconservation it would be easier to get rid ofit entirely.• Similarly, hibernation theory cannot explainwhy sleep deprivation can be fatal.ConclusionsBoth the restoration and the evolutionary approacheshave their advantages. However, the evidence doesseem to favour the restoration approach. That doesnot mean that evolution has had no impact on differentspecies’ sleep patterns. The example of the dolphinmakes this clear – because it lives in the water butbreathes air and needs sleep it has evolved a specialway of accommodating all these conflicting demands.www.psychlotron.org.ukContributed by Aidan Sammons