Sleep facilitates brain waste removal via glymphatic system, researchers confirm

The glymphatic system clears waste from the brain while people sleep, a process first learned about in 2012. Impaired clearance through this system may contribute to conditions such as Alzheimer’s and Parkinson’s disease.

In 2012, Jeffrey Iliff and colleagues injected a fluorescent tracer into the brains of living mice to study the system. The tracer chemicals were observed moving along the exterior of small arteries and small veins within the brain.

Astrocytes were noted to wrap around the blood vessels in the brain. Subsequently, Maiken Nedergaard and her team injected a green tracer into the cerebrospinal fluid of sleeping mice in 2013.

The flow of the tracer dropped by approximately 95 per cent after the mice were awakened. Furthermore, beta-amyloid was cleared roughly twice as quickly when the mice were asleep.

Natalie Hauglund and colleagues identified a specific control mechanism within mice. During non-REM sleep in mice, noradrenaline concentrations rose and fell at intervals of about 50 seconds.

Roslyn Bill at Aston University in Birmingham, UK, stated that glymphatic function declines with age due to stiffer arteries. Geir Ringstad and Per Kristian Eide demonstrated that the glymphatic system operates similarly in the human brain.

A study revealed that one night without sleep reduced tracer removal from people’s brains. Nedergaard found reduced glymphatic transport in mouse models of Alzheimer’s in 2016.

This January, Iliff and colleagues reported that the glymphatic system clears beta-amyloid and tau from people without dementia. Last year, a study found reduced glymphatic clearance in people with early psychosis when compared with controls.

Jeffrey Iliff cautioned that "The field is filled with promise, and we're just starting to do the studies that will find out if it's going to make good on that promise." Natalie Hauglund suggested that the glymphatic system could be an important factor in several diseases.

Near-infrared light can penetrate flesh and bone to reach deep into the brain. Bill noted that "we do not yet know that near-infrared light directly increases glymphatic flow in humans."