Dreaming LifeDreaming Life

Why does sleep deprivation have so many negative effects, ranging from irritability and the inability to concentrate to seizures, dementia and, death? Researchers studying waking and sleeping mice may have found the answer. Maiken Nedergaard of the University of Rochester Medical Center in New York and her colleagues have discovered that during sleep, the space between brain cells increases, so that fluid containing toxins can flow through these spaces. During sleep, toxins are removed from the brain.

In a study published in Science, the researchers looked at the glymphatic system, a pathway for cerebrospinal fluid (CSF). Waste from the central nervous system (CNS) passes through the glymphatic system, which plays the same role in the CNS as the lymphatic system does in other parts of the body.

Sleeping Woman, by Jacob Duck (c. 1600-1667)

Nedergaard's team, who discovered the glymphatic system, found that brain cells called glia control the flow of CSF through cells. Because moving CSF takes energy, the team thought it wouldn't be possible for the brain to transport CSF and process sensory information at the same time.

To test their hypothesis, they injected dye into the CSF of waking and unconscious mice. They found that when the mice were awake, the CSF hardly moved. However, when the mice were asleep or anesthetized, the CSF flowed rapidly.

When the researchers measured the spaces between brain cells, they found that it increased from 14% of brain volume when the mice were week to 23% of brain volume when they were asleep or anesthetized. When the mice were awake, CSF only flowed on the surface of the brain, but when they were awake, it flowed through the brain's deep tissues.

The team was able to increase the space between brain cells by injecting waking mice with drugs that block noradrenaline, and so induce unconsciousness. This showed that the sleep-wake state itself, rather than the circadian cycle, was controlling the amount of space.

Because neurodegenerative conditions like Alzheimer's disease are associated with the buildup of toxins in the brain, the researchers examined how sleep, or the lack of it, could affect such buildup.  They injected mice with beta amyloid, a protein that builds up in the brains of people with Alzheimer's. They discovered that the beta amyloid cleared from the brain twice as quickly when the mice were asleep as when they were awake.

Although this research explains one way that sleep can is good for us, it does not necessarily explain why sleep evolved in the first place.  Sleep has many other benefits. During deep NREM sleep, for example, your immune system is strengthened and damaged tissues are repaired, while memories are consolidated during REM sleep.  Sleep might have evolved to enable us to conserve energy or to keep us from moving and so avoid the attention of predators.

Original research: Xie, L., Kang, H., Xu, Q., Chen, M. J., Liao, Y., Thiyagarajan, M., ... & Nedergaard, M. (2013). Sleep Drives Metabolite Clearance from the Adult BrainSleep Drives Metabolite Clearance from the Adult Brain. Science, 342(6156), 373-377.