MARY LOUISE KELLY, HOST:
Researchers have long been puzzled by the relationship between sleep and Alzheimer's. New research may help solve that puzzle. NPR's Jon Hamilton reports on a study suggesting that the brain waves produced during deep sleep trigger a process that washes away toxins in our brains.
JON HAMILTON, BYLINE: People with Alzheimer's often have sleep problems, and sleep problems appear to make people more vulnerable to Alzheimer's. But Laura Lewis of Boston University says there's never been a good explanation for this connection.
LAURA LEWIS: You know, it's been known for a long time that sleep is really important for brain health, but why it is, you know, is more mysterious.
HAMILTON: Lewis was part of a team that wanted to solve the mystery, so they found a way to watch what was going on in the brains of 11 sleeping people. Lewis says one of the things they monitored was the liquid that flows through the brain and spinal cord. It's called cerebrospinal fluid, or CSF.
LEWIS: And that's when we discovered that during sleep, there are these really large, slow waves occurring maybe once every 20 seconds of CSF washing into the brain.
HAMILTON: Like the oscillations of a very slow washing machine. Earlier studies of animals had found that the flow of CSF increases during sleep and helps carry away waste products, including the toxins associated with Alzheimer's. But Lewis' team was able to see this process occur in the brains of people in real time, and that led to another discovery.
LEWIS: Before each wave of fluid, we would actually see a wave of electrical activity in the neurons. This electrical wave always happens first, and the CSF wave always seems to follow seconds later.
HAMILTON: Suggesting that the electrical wave was triggering each wash cycle. That brain wave was a very familiar one, called a slow wave. Slow waves appear when a person enters the state known as deep sleep, and Lewis says they play a role in both memory and brain disease.
LEWIS: It's already known that people with Alzheimer's disease have less of these electrophysiological slow waves, so they have smaller and fewer slow waves.
HAMILTON: Lewis' study, which appears in the journal Science, suggests that this reduction in slow waves is reducing wash cycles in the brain, and she says that would limit the brain's ability to clear out the toxins associated with Alzheimer's.
LEWIS: It would make sense that if there's large waves of fluid, of CSF, that that might in turn cause mixing and dispersion of all kinds of other fluids in the brain and help with this waste removal process.
HAMILTON: Lewis' team made one more discovery about sleeping brains. As the flow of cerebrospinal fluid increases, blood flow decreases. Less blood in the brain means more room for CSF to carry away waste. Lewis says the brain's self-cleaning system all seems to depend on getting the right kind of sleep.
LEWIS: Some disruption to the way sleep is working could potentially be contributing to the decline in brain health.
HAMILTON: Including Alzheimer's. William Jagust at the University of California Berkeley says the finding fits nicely with other research on sleep and Alzheimer's disease. Jagust was part of a team that studied the relationship between slow wave sleep and a toxin called beta-amyloid, which accumulates in the brains of people with Alzheimer's. He says they found something a bit disturbing.
WILLIAM JAGUST: It's a vicious cycle where amyloid decreases sleep and decreased sleep results in more beta-amyloid.
HAMILTON: Jagust says the new study suggests the increase in amyloid could be the result of less waste removal in the brain. He says it's likely that Alzheimer's, like heart disease, has more than one cause.
JAGUST: There are a bunch of things that are probably contributing to people's likelihood to getting Alzheimer's, and I think sleep is going to turn out to be one of them.
HAMILTON: Jon Hamilton, NPR News.
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