As far as a scientific answer to the question as to why do we sleep or why we need it, there is no satisfying answer to it as of now but there are various theories which try to answer this mysterious phenomenon, we call it 'sleep'.
In an average we spend about third of our life sleeping and yet we don't fully understand the importance of sleep, in fact there's so many things we don't know about sleep.
Body and Brain Tune-Up
We can't function without sleep, it is an anabolic and a building process that we go through everyday. It helps restoring the body’s energy supplies and clean out the build-up of brain junk.
Our body is like a car, you have to service it and change engine's oil, it just can't keep going forever. For a smooth and hassle free drive you maintain your car and service it to make sure it keeps running smooth, though it may keep running without service but the drive won't be smooth and it may break down as some stage. Similarly sleep can be thought of as a tune-up. After missing just one night's sleep you are likely to find yourself feeling cranky, exhausted, and unable to concentrate, poor decision making, memory impairment, reactions will slow, you may become forgetful and your vision can be affected. Not enough sleep can weaken the immune system making you susceptible to colds and other infections and can even increase your blood pressure.
Leading Theories
One of the theories is that sleep is a way to conserve energy and stay out of harm. Animals that sleep during a part of the day are less likely to have accidents or be eaten by predators. Also fewer calories are used while sleep, in humans that's an average of hundred calories, than being awake meaning that animals need to find less food each day compared to if they didn't sleep at all. Why sleeping during the night only? well it would be safer that way to keep calm, quite so if a predator stumbles upon you have better chance for escaping.
This theory may explain why having sleep is important but it doesn't explain all of the issues that are caused by missing out on getting enough sleep. Other researchers think that sleep is a chance for the body to repair itself and this seems quite believable as after having a good night sleep we feel better, fresh and active. However, as per this theory people who are more physically active would need more repair and hence should sleep more than those who are not much physically active, for instance a person who does labor job compared to person working in front of computer would require more sleep, although there is some evidence athletes sleep more after running but those who are completely inactive don't sleep any less either.
Brain Function
Most of the impairments seen in sleep deprivation are related to mind which suggests that sleep must be important for the brain to function properly. There are some evidences to suggest that the brain cells that are responsible for repair and restoration are more active during the sleep suggesting it is the brain in the body that needs time to recover rather than the other parts in the body.
In a recent study by scientists at the University of Rochester found that cerebral spinal fluid flows around the mice's brain ten times faster when they are asleep. This removes the toxins more efficiently, the researchers argue that this could explain the need for sleep and the problems that occur when we don't get enough of it.
Other researchers are not convinced as this is yet to be shown to happen in humans. Another theory suggests that sleep is required for memories to be stored properly. Researchers are still not clear as t why sleep is needed for it but studies have shown that people are good at remembering things that they have learned after a sleep. Although this might explain the difficulties related to brain functions that people could face who are deprived of sleep, it can't explain the physical symptoms.
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Conclusion
It is quite possible that none of the theories mentioned are correct or that more than one of the theories is required to explain why sleep is so important. One this is for sure, we need sleep for healthier life and happy life.
