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Why do we need sleep?


Why do we need sleep?

It is a fundamental question, one that has long gone unanswered, or at least not correctly answered. People have come up with many different reasons for why we need to sleep, from the sensible to the stupid. Here are five theories on why we sleep.

Brain washing

Despite the glib title this is actually a serious proposal. Recent research has shown that during sleep the brain is flushed of all the toxins that build up during the day. This is believed to be because while we are asleep our brains shrink a little bit and this allows the cleaning process to be done more easily and effectively. The research has been carried out on mice but is also supported by a range of other studies that have found people who have suffered from insomnia and other sleep disorders are more likely to suffer from Alzheimer’s and other neurological conditions. 

A study in 2012 by researchers at the University of Rochester first examined this phenomenon. They described the cleansing system of the brain, called the glymphatic system after the brain’s glia cells, and how it was able to wash all the toxic build up away. Following this in 2013 a team found that this cleaning was occurring during sleep and they even found that when the mice were injected with beta-amyloid protein, build up of which causes Alzheimer’s, it was removed far quicker when the mice were asleep than when they were awake. Thus, one reason we sleep could be that our brains need to be washed.

Memory storage

Reason number two that we sleep is that it may help us with our memory storage. The gist is that when we sleep we process all the day’s events, storing them so that they can be easily retrieved at a later date. There is a lot of evidence that backs this hypothesis up, with a wide range of studies showing that sleep is critical in the memory storage process.

In one study two groups were given the some memory task and then one group slept all night and the other had a purposely disturbed sleep. The group that had slept all night performed significantly better in the tests than those that had had a broken sleep. In another study the scientists have given the participants something they need to remember and then had them sleep in a laboratory. They are connected to an EEG machine that allows the scientists to see what sleep stage the participants are in and then they are woken during different stages and tested on their recall, as well as being tested the next day. In this research they found that people who had gone through their REM stage of sleep (where we dream the most) had better recall than those who had not gone through it.

Recharge the batteries

This is probably the oldest of all the theories, the one that most people think of immediately. It seems fairly obvious, by the end of the day your body and brain are tired, you have expended energy. After you sleep you feel awake again. Ergo sleep recharges the batteries. While some scientists believe that this is an outdated theory, new research has restored this theory. One study found that during the deeper stages of sleep the body releases a range of growth hormones that help to repair the body’s cells, these hormones help to rejuvenate all the cells after a busy day of operation. Another looked at how sleep helped people recover from illness and injury and found that it plays a huge role in the regeneration process, the study found that people who slept well each night were three times more likely to make a quick recovery from the common cold virus. One study kept rats awake for eight weeks and by the end of the period all the rats had developed large sores all over their bodies. These studies suggest that sleep is definitely helps us recharge our batteries.

Standby mode

Theory number four goes like this, when it is dark it is more dangerous for us to move around and less productive so rather than bump around at night risking death for no gain we sleep instead. It is a standby mode for animals. This is a theory that has not been able to come up with any hard evidence as yet but many in the scientific community think that it has viability. They think that while these other functions above are important, none are significant enough to explain why we actually need to sleep. The dolphin is able to survive without every fully sleeping (half of their brain sleeps at a time) and some think that humans will, with a few tweaks, one day be able to go without sleep. The standby mode is seen as an evolutionary advantage, it reduces the amount of energy used by an animal when the ability to hunt or scavenge is worse and ensures that they are safe when they may fall or hurt themselves.


The reality is that there may not be one single reason that we sleep. In fact, it seems more likely that all of the above are right, that we sleep because it helps us to wash our brains, that it provides a time when we can store memories, that helps us to recharge our batteries and that it is a useful standby mode which ensures we do not waste energy or hurt ourselves in the dark. In fact, it seems more likely that as we learn more about sleep, we will realise there are more reasons for sleeping.

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