Older people who get less deep sleep have higher levels of the brain protein tau, a sign of cognitive decline and Alzheimer's disease, according to a study.
Slow-wave sleep is the deep sleep people need to consolidate memories and wake up feeling refreshed, said researchers at the Washington University School of Medicine in the US.
The findings, published in the journal Science Translational Medicine, suggest that poor-quality sleep in later life could be a red flag for deteriorating brain health.
"What's interesting is that we saw this inverse relationship between decreased slow-wave sleep and more tau protein in people who were either cognitively normal or very mildly impaired, meaning that reduced slow-wave activity may be a marker for the transition between normal and impaired," said Brendan Lucey, an assistant professor at the Washington University.
"Measuring how people sleep may be a noninvasive way to screen for Alzheimer's disease before or just as people begin to develop problems with memory and thinking," Lucey said.
The brain changes that lead to Alzheimer's, a disease that affects an estimated 5.7 million Americans, start slowly and silently.
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Up to two decades before the characteristic symptoms of memory loss and confusion appear, amyloid beta protein begins to collect into plaques in the brain.
Tangles of tau appear later, followed by atrophy of key brain areas. Only then do people start showing unmistakable signs of cognitive decline.
The challenge is finding people on track to develop Alzheimer's before such brain changes undermine their ability to think clearly. For that, sleep may be a handy marker.
To better understand the link between sleep and Alzheimer's disease, Lucey, along with David Holtzman, a professor at Washington University, and colleagues studied 119 people 60 years of age or older.
Most -- 80 per cent -- were cognitively normal, and the remaining were very mildly impaired.
The researchers monitored the participants' sleep at home over the course of a normal week.
Participants were given a portable EEG monitor that strapped to their foreheads to measure their brain waves as they slept, as well as a wristwatch-like sensor that tracks body movement.
They also kept sleep logs, where they made note of both nighttime sleep sessions and daytime napping. Each participant produced at least two nights of data; some had as many as six.
The researchers also measured levels of amyloid beta and tau in the brain and in the cerebrospinal fluid that bathes the brain and spinal cord.
Thirty-eight people underwent PET brain scans for the two proteins, and 104 people underwent spinal taps to provide cerebrospinal fluid for analysis. Twenty-seven did both.
After controlling for factors such as sex, age and movements while sleeping, the researchers found that decreased slow-wave sleep coincided with higher levels of tau in the brain and a higher tau-to-amyloid ratio in the cerebrospinal fluid.
"The key is that it wasn't the total amount of sleep that was linked to tau, it was the slow-wave sleep, which reflects quality of sleep," Lucey said.
"The people with increased tau pathology were actually sleeping more at night and napping more in the day, but they weren't getting as good quality sleep," he said.
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