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You’ve been up since early morning, had a long day at school and it’s late in the day. Your eyes are tired, it’s hard to concentrate and you’re trying not to yawn. It sounds like you’re ready to sleep.

Humans spend a lot of their lives unconscious. Young babies spend more time asleep than awake, and as they grow older, people need less sleep. Most adults seem to need about eight hours per day. Sleep is important – without enough of it we feel tired, and our brains and bodies don’t function as well as if we’re well rested. It’s not just humans that need to sleep – it’s common across the animal kingdom.

The reason for sleep is still a bit of a mystery. We know lack of sleep makes us tired, but scientists aren’t sure why sleeping makes us feel better and more alert. Recent research may have discovered at least one reason why animals need to sleep.

The scientists were studying mice brains. They injected a dye into the cerebrospinal fluid (CSF) of the mice. As the name suggests, CSF is a fluid that surrounds the brain and spine of animals. The dye allowed the scientists to monitor the flow of CSF in the brain of awake and sleeping mice. They found that CSF flowed more readily in mice that were asleep, but barely flowed at all in mice that were awake. Further tests showed that when the mice slept, their brain cells shrank, allowing the CSF to flow.

The scientists also injected a substance called beta amyloid into the brains of the mice. Beta amyloid is a substance that builds up naturally in the brain. However, high levels can cause amyloid plaques, which in turn cause Alzheimer’s disease. They found that the sleeping mice flushed out the injected beta amyloid faster than those that were awake.

While the study was done on mice, the scientists think something similar could be happening in humans. They concluded that while you’re awake, junk like beta amyloid builds up. These wastes slow down your brain function, which you feel as tiredness. Going to sleep gives your brain a rest, allowing the CSF to get in there and tidy up.

While sleep probably also has other functions, it seems that one of its main roles is to turn off the lights so that the cleaners can get to work.

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