The exact reason why we sleep has long been one of the greatest mysteries of modern science. Many different theories have been proposed, but the fact is that no one is entirely sure why we spend roughly a third of our lives asleep.

There has been a great deal of research showing how sleep helps consolidate memories and repair the body, yet many scientists believe that these actions do not fully explain the underlying purpose of sleep, particularly from an evolutionary standpoint. Spending so much of our lives asleep and vulnerable opens us up to great danger, so many experts believe there must be a more compelling reason why we sleep.

One of the top theories of sleep suggests that slumber is necessary to allow the brain to clean up and reboot from the previous day’s activities. Research in mice supports this theory.

A study published in a 2013 issue of the journal Science revealed that sleep gives the brain a chance to clean itself. The study itself involved looking at the flow of fluids in the brains of mice in awake and sleep states.﻿﻿

The researchers focused in particular on how fluids flow within the lymphatic system or the spaces between neurons. This is something like a waste disposal system, clearing out the waste products that brain cells generate when performing normal tasks.﻿﻿

Fluid Flow Increases in Sleeping Brains

However, transporting these waste materials requires a great deal of energy, and the researchers hypothesized that the brain would not be able to support these cleaning functions and process sensory information at the same time. To test this idea, the study's lead author, Lulu Xie, spent two years training mice to fall asleep on a type of microscope that would allow the researchers to observe dye moving through living tissue.

After EEG activity confirmed that the mice were really asleep, a green dye was injected into their cerebrospinal fluid. A half-hour later the mice were awakened and a red dye was then injected. Through this process, the researchers were able to watch the movements of the green and red dye through the brain. What they observed was that while large amounts of cerebrospinal fluid flowed through the brain during sleep, the very little movement was observed while awake.﻿﻿

Spaces Between Brain Cells Become Larger During Sleep

So why was there such greater fluid flow during the sleeping states as opposed to the awake states? The researchers also observed that the interstitial spaces between brain cells became much larger during sleep, allowing fluid to flow more freely. These channels increased by approximately 60% during sleep.

The researchers also found when certain proteins were injected into the mice, the proteins were cleared away much more quickly during sleep. ﻿﻿

Possible Implications

“These findings have significant implications for treating ‘dirty brain’ disease like Alzheimer’s,” said Maiken Nedergaard, one of the study’s authors. “Understanding precisely how and when the brain activates the glymphatic system and clears waste is a critical first step in efforts to potentially modulate this system and make it work more efficiently.”

Scientists have long known that certain neurological conditions such as dementia, Alzheimer's, and stroke are all associated with sleep disturbances. According to Nedergaard, these results might suggest that lack of sleep could play a causal role in such conditions.

Now that the researchers have identified this brain-cleaning process, their hope is that it will lead to further research on how the process works and the possible role it might play in neurological conditions such as Alzheimer's disease.

The study also once again underscores the importance of sleep. "This could open a lot of debate for shift workers, who work during the nighttime,” Nedergaard told Science. "You probably develop damage if you don’t get your sleep."﻿﻿