By Stephen Beech
A good night’s sleep clears the mind like a “dishwasher” cleans crockery, new research suggests.
Deep sleep may wash away waste that builds up in the brain during waking hours, say scientists.
Their findings also offer insights into how sleeping pills may disrupt the “brainwashing” system, potentially affecting cognitive function in the long term.
Researchers already knew that the brain has a built-in waste removal process – called the “glymphatic system” – which circulates fluid in the brain and spinal cord to clear out waste.
The process helps remove toxic proteins that form sticky plaques linked to neurological disorders, such as Alzheimer’s disease.
But what drives the system has remained unclear, until now.
Danish scientists found that a molecule called norepinephrine plays a key role in brain cleaning in mice.
During deep sleep, the brainstem releases tiny waves of norepinephrine about once every 50 seconds.
Study senior author Dr. Maiken Nedergaard, of the University of Rochester in New York, and the University of Copenhagen in Denmark, explained that norepinephrine triggers blood vessels to contract, generating slow pulsations that create a rhythmic flow in the surrounding fluid to carry waste away.
She said: “It’s like turning on the dishwasher before you go to bed and waking up with a clean brain.
“We’re essentially asking what drives this process and trying to define restorative sleep based on glymphatic clearance.”
To find clues, Dr. Nedergaard and her team looked into what happens in mice when the brain sleeps.
They focused on the relationship between norepinephrine and blood flow during deep sleep.
They found that norepinephrine waves correlate to variations in brain blood volume, suggesting norepinephrine triggers a rhythmic pulsation in the blood vessels.
The researchers then compared the changes in blood volume to brain fluid flow.
They found that the brain fluid flow fluctuates in correspondence to blood volume changes, suggesting that the vessels act as pumps to propel the surrounding brain fluid to flush out waste.
Study lead author Dr. Natalie Hauglund, of the University of Copenhagen and the University of Oxford, said: “You can view norepinephrine as this conductor of an orchestra.
“There’s a harmony in the constriction and dilation of the arteries, which then drives the cerebrospinal fluid through the brain to remove the waste products.”
Dr. Hauglund then wanted to know if all sleep is created equal.
To find out, the research team gave zolpidem, a common drug to aid sleep, to mice.
They found that the norepinephrine waves during deep sleep were 50% lower in zolpidem-treated mice than in naturally sleeping mice.
Although the zolpidem-treated mice fell asleep faster, fluid transport into the brain dropped more than 30%.
The researchers say their findings, published in the journal Cell, suggest that the sleeping aid may disrupt the norepinephrine-driven waste clearance during sleep.
Dr. Hauglund said: “More and more people are using sleep medication, and it’s really important to know if that’s healthy sleep.
“If people aren’t getting the full benefits of sleep, they should be aware of that so they can make informed decisions.”
The research team says that the findings likely apply to humans, who also have a glymphatic system, although it requires further testing.
Scientists have observed similar norepinephrine waves, blood flow patterns and brain fluid flux in humans.
Their findings may offer insights into how poor sleep may contribute to neurological disorders such as Alzheimer’s.
Dr. Nedergaard added: “Now we know norepinephrine is driving the cleaning of the brain, we may figure out how to get people a long and restorative sleep.”
