Deep Sleep Cleans Your Brain, Scientists Discover

In a new study pioneered at Boston University, a team of researchers has discovered how sleep cleans toxins from the brain. The study was led by Assistant Professor of Biomedical Engineering Laura Lewis and explored how synchronised brain waves released during non-REM sleep could play a role in preventing build-up of toxins in the brain.

The findings were published in the journal Science and could unlock new insight into the mechanics of neurodegenerative diseases such as Alzheimer’s. Lewis hopes the results could help create new ways to treat and prevent the debilitating conditions that degenerate nerve cells and have a negative impact on cognitive function.

The brain cleansing properties of non-REM sleep

During sleep, the brain progresses through several phases, ranging from light rest to a deep slumber that resembles unconsciousness. Dreams mostly occur during rapid eye movement (REM) sleep, a phase that generally occurs several times throughout the night. Lewis and her colleagues focused on non-REM sleep, the deep phase that usually takes place earlier in the night and is linked to memory retention.

The Boston University study built on previous research conducted on mice, which showed that sleep flushed toxins such as beta amyloid, which is linked to Alzheimer’s disease. Lewis and her team were interested in how the toxins are cleansed out and why the process occurs only during sleep. She hypothesised that the cleanse involved cerebrospinal fluid, a thin and transparent liquid that flows around the brain. To investigate the mechanics of the process, the team designed a study to test several different variables.

Leveraging electroencephalography and MRIs

Participants were fitted with an electroencephalography (EEG) cap and asked to fall asleep inside an MRI machine. Lewis and the team then analysed electrical currents in their brains and compared this to the aligning sleep stages. They also measured blood oxygen levels in the brain, as well as the level of cerebrospinal fluid flowing in and out of the organ.

“We had a sense each of these metrics was important, but how they change during sleep and how they relate to each other during sleep was uncharted territory for us,” says Lewis.

The researchers found that large, slow moving waves of cerebrospinal fluid washed over the brain during non-REM sleep. The corresponding EEG readings revealed that at the same time, neurons start to synchronise activity.

“First you would see this electrical wave where all the neurons would go quiet,” says Lewis. When all neurons temporarily stopped firing the need for oxygen was reduced, which limited blood flow to the brain. This freed up space for cerebrospinal fluid to gush in, circulate around the brain and flush out accumulated metabolic by products such as beta amyloid.

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