Alzheimer's Disease is one - very common - form of senile dementia. It usually affects older people and progressively robs them of their mental faculties. It occurs because a protein called beta amyloid builds up in the brain forming harmful deposits known as plaques, which are toxic to nerve cells.
What's confusing though is that the brain naturally makes beta amyloid all the time but seems to wash away most of it when we go to sleep. But what aspect of sleep is important for this clean-up process, and what are the implications for people who are chronically poor sleepers?
At Washington University, in St Louis, Yo-El Ju recruited a small group comprising 22 volunteers who agreed to having a computer eavesdrop on their brainwaves while the slept.
They spent two nights in a sleep lab. One occasion was just a control visit when they were allowed to sleep normally. But on a second occasion, the computer was programmed to spot signature patterns of neurological activity corresponding to so-called "slow wave sleep".
This is the deepest phase of sleep and occurs shortly after we first nod off. Whenever the computer detected that a subject was entering slow wave sleep, the computer delivered a succession of "peeping" sounds through earphones. These sounds escalated in volume until the brain wave activity confirmed that the subject had been sufficiently roused to arrest slow-wave sleep without waking them up.
"We weren't trying to sleep deprive the volunteers," explains Ju, "but instead to rob them selectively of slow-wave sleep. Each candidate received about 1200 auditory stimuli across the night."
On both occasions, during the following morning, the volunteers underwent a spinal tap to collect a sample of the cerebrospinal fluid that surrounds the brain. This was tested for levels of beta-amyloid, the Alzheimer-linked protein secreted by nerve cells, as well as several other proteins made by other brain cells.
Preventing slow-wave sleep, the team found, was associated with a significant increase in beta-amyloid concentrations in the cerebrospinal fluid.
"But this was not mirrored in the other proteins we measured," says Ju. "What that shows is that beta-amyloid isn't accumulating because the brain is failing to wash it away - otherwise the other proteins would all be higher too. Instead, this is increased production of beta-amyloid, probably because brain cells are becoming more active when slow wave sleep is disturbed."
Asked why this finding matters, Ju points out that sleep disorders, and the use of sleep aids such as drugs, carry an increased risk of developing Alzheimer's Disease.
"But if we know what phase of sleep is playing a critical role in changing that risk, then we have a much better chance of finding ways to stop it."
Ju does also emphasise that the occasional disturbed night is not going to give the average person Alzheimer's.
"It's the chronic, sustained sleep disturbance that happens over many years that does that!"
This is something Ju herself finds highly reassuring: "I'm 7 months pregnant and about to experience a significant amount of sleep deprivation... but hopefully not for too long!"
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