Sleep clears out waste produced during the day

Although it is common knowledge that a good night’s rest does us good, the exact biological function of sleep has remained mysterious. A team of New York-based researchers have proposed that the role of sleep may be to facilitate removal of potentially harmful waste produced by our brain cells during the course of the day. The interstitial fluid – the fluid surrounding our cells – is drained in the rest of the body by the lymphatics. However, by contrast the brain relies on a network of vessels known as the glymphatic system. The fluid in our brains flows through channels formed by glial cells, which link up with the spinal column so that it is gradually exchanged with the cerebrospinal fluid (CSF).

The team used two-photon imaging (a type of microscopy where a fluorescent molecules is excited by two low-energy photons simultaneously) in live mice to follow the movement of a fluorescent tracer molecule through the brain. They infused the tracer into the CSF of mice and found that the rate at which the tracer entered the brain was significantly higher in sleeping and anaesthetised mice than in awake animals. The researchers hypothesised that this was due to an increase in the volume of the interstitial space, which would therefore allow more ready convective flow.

To test this hypothesis, they recorded interstitial volume fraction of the mice under different sleep/wake conditions. The results showed that the interstitial space volume fraction was markedly larger in sleeping and anaesthetised mice than in those that were awake.  The researchers went on to show in the same model that Aβ (a protein produced by neurons associated with degenerative disorders) is cleared from the brain more efficiently during sleep than wakefulness, suggesting that removal of dangerous cellular waste to prevent neuronal damage could be a key function of sleep.

http://www.sciencemag.org/content/342/6156/373