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 Theories of Why We Sleep:

Scientists have explored the question of why we sleep from many different angles. They have examined, for example, what happens when humans or other animals are deprived of sleep. In other studies, they have looked at sleep patterns in a variety of organisms to see if similarities or differences among species might reveal something about sleep's functions. Yet, despite decades of research and many discoveries about other aspects of sleep, the question of why we sleep has been difficult to answer. The lack of a clear answers to this challenging question does not mean that this research has been in vain. In fact, we now know much more about the function of sleep, and scientists have developed several promising theories to explain why we sleep. In light of the evidence they have gathered, it seems likely that no single theory will ever be proven correct. Instead, we may find that sleep is explained by two or more of these explanations. The hope is that by better understanding why we sleep, we will learn to respect sleep's functions more and enjoy the health benefits it affords.

Inactivity Theory

One of the earliest theories of sleep, is the adaptive or evolutionary theory, inactivity at night - adaptation - serves as survival function - keeps organisms out of harm's way at times when they would be particularly vulnerable. Animals that are able to stay still during these periods of vulnerability have an advantage over other animals that remain active. These animals do not have accidents during activities in the dark - are not killed by predators. Through natural selection, this behavioral strategy became what we now recognize as sleep. A counter-argument to this theory - it is always safer to remain conscious to be able to react to an emergency (even if lying still in the dark at night). So there does not seem to be any advantage of being unconscious and asleep if it comes to  safety.

Energy Conservation Theory

One of strongest factors in natural selection - competition for and effective utilization of energy resources. Less apparent to people living in societies in which food sources are plentiful. Primary function of sleep - reduce individual's energy demand and xpenditure during part of day or night, (especially at times when it is least efficient to search for food). Research shows reduction in energy metabolism during sleep (10 percent in humans, more in other species). E.g, both body temperature and caloric demand decrease during sleep, compared to wakefulness. Such evidence supports proposition that one of primary functions of sleep - to help organisms conserve their energy resources. Many scientists consider this theory to be related to, and part of, the inactivity theory.

Restorative Theories

Another explanation for why we sleep is based on the long-held belief that sleep in some way serves to "restore" what is lost in the body while we are awake. Sleep provides the opportunity for the body to repair and rejuvenate itself. In recent years, these ideas have gained support from empirical evidence collected in human and animal studies. Most striking of these - animals deprived entirely of sleep lose all immune function and die in a matter of weeks - further supported by findings that many of the major restorative functions in the body like muscle growth, tissue repair, protein synthesis, and growth hormone release occur mostly, or in some cases only, during sleep. Other rejuvenating aspects of sleep are specific to brain and cognitive function. E.g, while awake, the neurons in the brain produce adenosine, a by-product of the cells' activities. Build-up of adenosine in the brain - thought to be one factor that leads to perception of being tired. (Incidentally, this feeling counteracted by use of caffeine, which blocks actions of adenosine in brain - keeps us alert.) Scientists think that this build-up of adenosine during wakefulness may promote "drive to sleep." As long as we are awake, adenosine accumulates and remains high. During sleep, the  body has a chance to clear adenosine from the system - more alert when awake.

Brain Plasticity Theory

Most recent and compelling explanation - based on findings that sleep is related to changes in structure and organization of brain. This phenomenon, known as brain plasticity - not entirely understood - but connection to sleep has several implications. E.g it becomes clear that sleep plays a role in brain development in infants and young children. Infants spend about 13 to 14 hours per day sleeping. About half of that time is spent in REM sleep - the stage in which most dreams occur. The link between sleep and brain plasticity becomes clear in adults as well. This is seen in the effect that sleep and sleep deprivation have on people's ability to learn and perform a variety of tasks. Although these theories remain unproven, science has made a tremendous stride to discover what happens during sleep, what mechanisms in the body control cycles of sleep and wakefulness. While this research does not directly answer the question, "Why do we sleep?" it sets the stage to put the question in a new context and generate new knowledge about this essential part of life.

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