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Sleep is one of our most basic human needs. We spend about a third of our lives doing it and there are serious health consequences to sleep deprivation. It’s safe to say that sleep is as necessary as food and water, but why do we all spend so many hours doing it?

Scientists simply aren’t sure why. Sleep researcher William Dement, who co-discovered REM sleep and is often described as the father of sleep medicine, told National Geographic that, “as far as I know, the only reason we need to sleep that is really, really solid is because we get sleepy.”

As lying unconscious for an extended period of time made our ancestors vulnerable to attacks, researchers suggest that there must be some advantages to outweigh this considerable risk. The biggest hurdle to understanding why we sleep was that the brain activity that occurs during sleep was largely hidden from researchers. New methods, though still somewhat in their infancy, are helping researchers to better understand the purpose of sleep.

In a 2000 study, published in Brain Research, researchers Terrence Sejnowski and Alain Destexhe, added to the growing evidence that sleep allows us to “consolidate” our memories. Consolidation is the important process whereby short-term memory is converted into long-term memory. Sejnowski and Destexhe found that when we sleep, our brain consolidates the information that we’ve learnt in a day by opening calcium-mediated biochemical pathways in pyramidal neurons. Researchers suggest that this process and the necessary “network reorganization” take time and need to occur when normal processes such as sensory processing aren’t happening. “This may ultimately be the primary reason why we need to sleep,” researchers note.

Some researchers suggest that the benefits of memory consolidation don’t outweigh the risks of sleeping. They instead point to other theories. Researchers from the University of Rochester argue that sleep may provide the brain with an important opportunity to take out the trash. In a 2013 study, published in the journal Science, researchers found that cerebral spinal fluid is pushed around the brain to clear waste chemicals that are produced as part of a cell’s natural activity. This method of waste removal, known as the glymphatic system, mainly occurs when we sleep.

"You can think of it like having a house party. You can either entertain the guests or clean up the house, but you can't really do both at the same time," lead researcher Dr Maiken Nedergaard told the BBC.

Another theory suggests that the reason why we sleep is down to energy demands. In a 2008 study, published in PLOS Biology, Emmanuel Mignot says that this theory is firmly grounded in natural selection. As animal performance and prey availability peak at different times during the day, sleep could have been selected for to save energy. Mignot suggests that there are some flaws in this theory because it does not explain the selection for REM sleep, which in most species results in an increase in energy expenditure.

There are of course limitations to all the main theories put forward to explain the mystery of why we sleep, but an improvement in technology and a renewed interest in this question might mean a definitive answer won’t remain elusive for long. For now though, why don’t we all go and sleep on it?

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