Psychological research into why we sleep can be separated into two distinct methods of thought – restorative and adaptive theories. These two broad points of view have dominated sleep research for past two decades. In this essay, the specifics of the above points of view will be discussed, as well as actual psychological theories as to why we sleep.
Restorative theories are based on the idea that we sleep so that physiological and biochemical repairs can take place. This implies that without sufficient sleep, a human being’s health, both physical and mental, will deteriorate. During sleep, three essential processes have been proposed:
•	The need to breakdown an unwanted substance which accumulates in the body during activity
•	The need to carry out some essential process of chemical synthesis, which is inefficient or impossible during wakefulness
•	To allow the recovery of neural components or pathways which fatigue during arousal
Oswald (1974) proposes that body restoration occurs during S-sleep, and that brain restoration occurs during D-sleep, partially through stimulating neural protein synthesis. Both of these periods occur during REM (Rapid Eye Movement) sleep, a crucial time in which we dream and reorganize our memories/resolve issues of the sub-conscious.
Restorative theories also encompass learning and developmental theories. Learning theories state that our brains reorganize and store information during REM sleep, and developmental theories state that REM sleep assists in developing our brains and essentially teaching us how to behave/survive.
Adaptive theories are based on the idea that sleep is a useful behavior which keeps an organism out of harms way, both from predators and inefficient energy expenditure. Adaptive theorists do not believe that sleep has restorative properties, but is simply a natural evolutionary activity. These theories rest upon three assumptions:
•	Sleeping involves reduced energy expenditure
•	Immobility affords security against predators
•	Sleep occurs during periods of the circadian cycle when an organism is least competent to be active
Of all sleep theories, adaptive theories are considered the weakest due to the fact that they fail to explain the proven stages of sleep, and merely disregard them. However, there is seemingly damning evidence against restorative theories. Meddis (1975) showed that shrews, swifts and Dall porpoises survive without any sleep whatsoever, hence contradicting the very principles that support restorative theories.
Horne (1988) proposed a theory that encompasses elements from both restorative and adaptive theories known as the Core-Optional theory. According to Horne’s theory, sleep is composed of two parts – Core and Optional sleep. Core sleep is considered to take place during the first three sleep cycles and is required by the organism to perform essential brain processes, including the recovery and restitution of neural and related tissue.
Optional sleep is thought to take place during half of D-sleep and much of stage 2 sleep. Horne believes Optional sleep to be disposable and non-essential, considering it to merely satisfy some form of behavioural drive, to occupy unproductive hours and in the case of small mammals, to conserve energy.
Since fossil evidence fails to show the development of sleep patterns, there is an inherent lack of evidence to support/contradict adaptive theories, however there is significant evidence against restorative theories. Several biochemical and neurological studies have been undertaken to determine what actually induces sleep.
A natural sleep-inducing chemical released in the brain is yet to be found, the results proving to be inconclusive. The proposal that growth hormone mediates restorative processes is now in significant doubt. Clugston and Garlick (1982) concluded that protein is built up during the day and not during sleep, contradicting Oswald’s restorative theories in relation to protein synthesis processes during D-sleep.
Sleep deprivation studies have been undertaken in an attempt to prove that an animal’s physiological and psychological health will deteriorate if they have not slept for prolonged periods of time. Meddis (1983) studied humans that had not slept for several days, only to find that there was no physiological damage. However, psychological damage was present, with all the subjects reporting lack of prolonged concentration and some perceptual distortions.
Meddis concluded that sleep deprivation was not devastating to humans, and that the psychological effects were only due to a disruption in the mechanisms that control sleep. Adding to this, there have been several reports of insomniacs showing no signs of physiological and/or psychological effects due to sleep deprivation.
Lavie (1984) described the case of a man who had sustained a shrapnel injury to the Pons, hence losing his ability to undergo D-sleep. This man showed no obvious health or learning problems that should’ve been present according to several restorative and learning theories.
Only one learning theory could accommodate Lavie’s findings, and that was Roffwarg’s Ontogenetic Hypothesis (1966). Roffwarg states that D-sleep is crucial for the cerebral development of very young mammals, even foetuses. However, as we age, D-sleep decreases since many essential processes have already taken place in our early development.
Although several purely restorative theories have received many fatal blows, the connection between D-sleep and learning has been firmly established by many studies. Bloch (1977) showed that D-sleep increased by 10-15% over six days while the rats were engaged in a complex maze task. More importantly, it decreased when the task was complete.
Marczynski (1992) used intracellular electrical recordings in freely moving cats to determine neuronal firing patterns during learning tasks and sleep. They confirmed that random firing patterns during both S-sleep and D-sleep removed unwanted data and strengthened the cat's learning.
Oswald’s restorative theory received yet another fatal blow with the findings from Ryback’s and Adey’s studies. Ryback (1971) conducted a study in which several people were completely rested for 6 weeks. Adey (1968) conducted a similar study on quadriplegics. According to Oswald’s theory, in both cases S-sleep should decrease due to lowered levels of activity; however, no subject in either study showed lowered levels of S-sleep.
Horne provided new insight into exercise-induced S-sleep, conducting two studies in which subjects were forced to exercise while being sprayed by hot or cold water (Horne & Staff, 1983; Horne & Moore, 1985). Horne concluded that it was not an increase in activity that heightened periods of S-sleep, but a raise in body temperature by a few degrees Celsius, thus causing a rise in brain temperature and consequently leading to an increased metabolic rate. Alternatively, these rises in temperature could be merely disrupting sleep control mechanisms; however no studies have been undertaken to validate this theory.
Comparative evidence is used to assess sleep theories by comparing the sleeping patterns of different animals and comparing the two animals’ form, behaviour and environment. However, Horne states that these comparissions do not shed much light on why humans sleep, due to the fact that why humans sleep and why birds sleep could potentially have completely different answers.
Comparative studies are yet another source of strong evidence against restorative theories; however they are practically the only source of evidence that support adaptive theories. This comes from the fact that although animals may have very similar sizes and complexity, their amounts of sleep can vary enormously. Allison and Van Twyler (1970) showed that bats sleep for 20 hours, however shrews hardly sleep at all, if ever.
However, comparative studies are not all supportive of adaptive theories. In several cases, sleep appears to be evolutionarily maladaptive, for example Pilleri’s (1979) study into Indus dolphins. Pilleri showed that this particular breed of dolphin sleeps for several minutes at a time over the course of the day. This behasviour would surely be of a disadvantageous nature to the dolphin, considering that it must keep swimming to avoid dangerous currents and debris.
Another odd case is that of the Bottlenose dolphins and porpoise’s sleeping habits. Mukhametov (1984) showed that the two cerebral hemispheres of bottlenose dolphins sleep independently. From studies like these, questions must be raised. If sleep has no purpose/advantages to some animals, why has it not been evolutionarily deselected? In the case of the bottlenose dolphin, why have such a complex mechanism if sleep has no restorative properties?
In conclusion, serious research into why we sleep has only been going on for 50 years, and is still relatively undeveloped. As more complexities about why we sleep become apparent, usually through new measurement techniques, theories must adapt or die. Unfortunately, most theories have been dealt fatal blows; however adaptive theories and the Ontogenetic Hypothesis have stood the test of time. During the last decade, functional research into why we sleep has declined; therefore it looks unlikely that we will solve the mysteries and complexities of sleep in the near future.