CHICAGO, Jan. 10 (Xinhua) -- Researchers at Washington University School of Medicine in St. Louis found that older people who have less slow-wave sleep, the deep sleep one needs to consolidate memories and wake up feeling refreshed, have higher levels of the brain protein tau, and elevated tau is a sign of Alzheimer's disease and has been linked to brain damage and cognitive decline.
To better understand the link between sleep and Alzheimer's disease, the researchers studied 119 people aged above 60 or older, of which 80 percent were cognitively normal, and the remainder 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; 104 people underwent spinal taps to provide cerebrospinal fluid for analysis; and 27 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," said first author Brendan Lucey, an assistant professor of neurology and director of the Washington University Sleep Medicine Center. "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."
The brain changes that lead to Alzheimer's, a disease that affects an estimated 5.7 million Americans, start slowly and silently. It takes up to two decades before the characteristic symptoms of memory loss and confusion appear, and amyloid beta protein begins to collect into plaques in the brain.
"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.
Nonetheless, the researchers don't expect sleep monitoring to replace brain scans or cerebrospinal fluid analysis for identifying early signs of Alzheimer's disease, though it could supplement them.
The findings were published on Jan. 9 in Science Translational Medicine.
