Here
are a few new clues about this nightly cleanse.
By Kat Eschner November 1,
2019
Rapid Eye Movement (REM) sleep gets
the most hype, but it's far from the only important part of our snooze cycle.
A new study shows that slow-wave sleep (SWS), the
deepest phase of non-REM sleep, may be the key to detoxing our
brains.
The study identified a
cycle that occurred during SWS in 11 participants, who were all healthy people
between the ages of 20 and 33. Animal research in the 1970s found that
"slow waves" of neuronal activity sweep across the brain during SWS.
Then, blood volume decreases and cerebrospinal fluid sweeps in to fill the new
space. Cerebrospinal fluid is known to play an
important role in flushing out toxic waste that can cause neurodegeneration,
and the researchers think that these waves of it are an important part of the
brain's "wash-rinse" cycle.
During this process,
cerebrospinal fluid “started to flow upwards in these really large, slow
waves... much larger than we’d ever seen when were scanning the awake brain,”
says study senior author Laura Lewis of Boston University.
To do the research, the
team had its study subjects come to the lab at midnight, after only sleeping
for four hours the previous evening. (“We talk so much about how important
sleep is,” Lewis said, “but you really tax your own sleep when you’re trying to
do all of your experiments in the midnight to 3 a.m. zone.”) They outfitted
them with electrode-covered caps to measure electrical activity, then used an
accelerated MRI technique to snap images every 360 milliseconds.
The cycle those rapid
snapshots revealed—first a “slow wave” of electrical activity in the subjects’
brain cells, then a dip in brain blood oxygenation, followed by an upward rush
of cerebrospinal fluid—is something that hadn’t previously been identified in
humans. But it seems to happen about every 20 seconds during SWS.
Because previous work has
shown that sleep plays an important role in clearing the brain of waste
proteins that are known to cause
neurodegeneration if they hang around too long, and cerebrospinal fluid is
known to cleanse these proteins, Lewis and her colleagues think the
cerebrospinal fluid wave is helping to wash the brain of waste.
However, they didn’t
directly measure the rate at which toxins were fleeing the scene—so this study
doesn’t go so far as to prove that SWS removes the bulk of these dangerous
proteins. Lewis’s team is working on that research now, along with expanding
their study to include adults in their 60s and 70s, to understand how aging
might affect this wash-rinse cycle.
In the longer term, better
understanding the dynamics of slow-wave sleep may be able to help
us treat neurodegenerative diseases associated with aging, such
as Alzheimer's.
But Yo-El Ju, a neurologist who studies sleep at the Washington University
School of Medicine and who was not involved in the current study, says this
might only show part of the picture. "It's important to keep in mind that
this may be just one of the ways in which sleep interacts with brain metabolism
and waste clearance," she says. Other systems that the MRI was not
designed to measure—such as the glymphatic
system—may also play a crucial role. We have
a lot left to learn about what sleep does for us. For now, one thing is
certainly clear: Sleep
is incredibly important, and most of us aren't getting enough.
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