The purpose of sleep remains unknown. Using state-of-the-art in vivo two-photon imaging to directly compare two arousal states in the same mouse, Xie et al., in a paper titled “Sleep Drives Metabolite Clearance from the Adult Brain” published in Science, found that metabolic waste products of neural activity were cleared out of the sleeping brain at a faster rate than during the awake state. This finding suggests an explanation for how sleep serves a restorative function, in addition to its well-described effects on memory consolidation.
From the abstract:
The conservation of sleep across all animal species suggests that sleep serves a vital function. We here report that sleep has a critical function in ensuring metabolic homeostasis. Using real-time assessments of tetramethylammonium diffusion and two-photon imaging in live mice, we show that natural sleep or anesthesia are associated with a 60% increase in the interstitial space, resulting in a striking increase in convective exchange of cerebrospinal fluid with interstitial fluid. In turn, convective fluxes of interstitial fluid increased the rate of β-amyloid clearance during sleep. Thus, the restorative function of sleep may be a consequence of the enhanced removal of potentially neurotoxic waste products that accumulate in the awake central nervous system.
So: we sleep to clean our brains (as the authors of the paper want us to believe).
This research is tentative, as it’s only been found to be the case in mice. The Guardian provides more commentary:
Maiken Nedergaard, who led the study at the University of Rochester, said the discovery might explain why sleep is crucial for all living organisms. “I think we have discovered why we sleep,” Nedergaard said. “We sleep to clean our brains.”
Writing in the journal Science, Nedergaard describes how brain cells in mice shrank when they slept, making the space between them on average 60% greater. This made the cerebral spinal fluid in the animals’ brains flow ten times faster than when the mice were awake.
The scientists then checked how well mice cleared toxins from their brains by injecting traces of proteins that are implicated in Alzheimer’s disease. These amyloid beta proteins were removed faster from the brains of sleeping mice, they found.
Nedergaard believes the clean-up process is more active during sleep because it takes too much energy to pump fluid around the brain when awake. “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,” she said in a statement.
According to the scientist, the cerebral spinal fluid flushes the brain’s waste products into what she calls the “glymphatic system” which carries it down through the body and ultimately to the liver where it is broken down.
The importance of replicating this work is obvious, lest it be considered cargo cult science.