Every single living thing in the world that we know sleep, from the laziest animal on earth, such as the brown bat who sleeps approximately 19.9 hours in 24, to the one that sleeps barely none, such as the insomniac giraffe, who sleeps about 1.4 hours out of 24. Even plants have a circadian rhythm similar to fauna. However, it is a hot topic between psychologists, with differing theories battling it out to "prove" once and for all that it is them that are right. This essay aims to outline two theories very briefly and to describe and evaluate the other two using research that have been made into the subject.
One theory that is holding strong in the proverbial battleground is that of conservation. The basic principle here is that sleeps allows us to conserve energy when it is least needed, i.e. at night, as we have not adapted to hunt or shop in pure darkness, unlike other nocturnal animals such as the hedgehog or fox. Hibernation is a form of evidence for this theory, as the animals that do hibernate seems to have adapted to steer clear of the cold weather and to laze in bed all winter before emerging out once more in the spring. This, the psychologists say conserves the energy they would have wasted in their futile search for food. Furthermore, Berger and Phillips (1984) found a definite decrease in human body temperature when the participants fell asleep, which they claim to be fair evidence supporting this theory.
On the other hand, there lies the theory of Learning Promotion which, like all of the theories holds steady land in the combat art of psychology. This theory is one that believes that sleep is the key to learning, and that all information or experiences during the day is not properly processed or absorbed, and that sleep sorts out the good and bad and rids the brain of the un-necessary information gathered during the day. This theory also holds that sleep affects memory and storage, which is shown by Smith and Rose (1997), where they found that rats forgot the location of a hidden platform in milky water, in which they were immersed when they were deprived of REM sleep.
The third theory that has a lot to say for itself, is that of restoration, which in its elemental form means that as we go about our daily lives we drain ourselves physically and psychologically and in effect disrupt the body's natural and preferred state of homeostasis. Sleep, according to the psychologists fighting this theory out, remedies this balance and replenishes the body's energy after mental or physical efforts. This theory is split into two, with research supporting the psychological restoration side, and others in regards to the physical repair aspect.
Hicks and Garcia (1987) found solid evidence that participants that were exposed to mild stressors seemed to sleep for longer periods than a control group. However this is cast into doubt when one considers the findings of Kales (1974), in which they found that insomniacs had high levels of stress. To reiterate with different research, Hartmann (1973) and Rosch (1996) found the same results, where Hartmann found that humans slept much more during times of mild stress whilst Rosch in 1996 found that severe stress led to insomnia. The weakness of all these research is the vagueness in which they were tested. How stressful is "mild stress"? How much sleep must be lost for "insomnia"? However the findings do suggest that stress affects sleep in one way or another, with some stress resulting in a need for restoration, psychologically, whilst too much leads directly to the incapability or severe difficulty to sleep.
On the physical side of the theory, the researchers explore the reason why we sleep more when we are ill or have ran a long distance. Stern and Morgane (1974) suggested that neurotransmitters may be restored during sleep, or more specifically, REM sleep, although no solid evidence for this exist. Shapiro et al (1981) studied participants that had taken part in an ultra-marathon slept for an hour and a half longer for two days later, with a significant increase in stage 4 sleep, and a decrease in REM sleep. This seems to suggest that stage 4 sleep is of a restorative variety as opposed to the other stages. Furthermore, Empson (1989) found that a lack of stage 4 sleep resulted in symptoms similar to fibrositis, e.g. inflammation of the back muscles, which manifests themselves as pain and stiffness. This is good evidence that a link exists between physical effects and stage 4 sleeps, which of course like all above previous research