Getting the right amount of sleep — not too much and not too little — could reduce your risk of mental decline as you age, even if you have early Alzheimer's disease, a new study claims.
Poor sleep and Alzheimer's disease are both associated with thinking ("cognitive") declines, but separating out the effects of each has been a challenge.
This new study included 100 older adults whose cognitive function had been monitored for an average of 4.5 years. The participants underwent a sleep study at an average age of 75 and were tested for the high-risk Alzheimer's genetic variant APOE4 and for levels of Alzheimer's proteins in their cerebrospinal fluid.
Most of the participants (88) had no cognitive impairment, 11 had very mild impairment and one had mild impairment.
Overall, cognitive scores declined for those with less than 5.5 or more than 7.5 hours of self-reported sleep per night, while scores remained stable for those in the middle of the range.
"It was particularly interesting to see that not only those with short amounts of sleep but also those with long amounts of sleep had more cognitive decline," said co-senior study author Dr. David Holtzman. He is a professor of neurology at Washington University School of Medicine in St. Louis. "It suggests that sleep quality may be key, as opposed to simply total sleep."
This U-shaped link between sleep and mental decline held true after the researchers adjusted for factors that can affect both sleep and cognition, such as age, sex, levels of Alzheimer's proteins, and the presence of APOE4, according to the study published Oct. 20 in the journal Brain.
The findings could help keep people's minds sharp as they age, the researchers suggested.
"It's been challenging to determine how sleep and different stages of Alzheimer's disease are related, but that's what you need to know to start designing interventions," said study first author Dr. Brendan Lucey, director of Washington University's Sleep Medicine Center.
"Our study suggests that there is a middle range, or 'sweet spot,' for total sleep time, where cognitive performance was stable over time. Short and long sleep times were associated with worse cognitive performance, perhaps due to insufficient sleep or poor sleep quality," Lucey said in a university news release. "An unanswered question is if we can intervene to improve sleep, such as increasing sleep time for short sleepers by an hour or so, would that have a positive effect on their cognitive performance so they no longer decline?"
More research is needed to answer that question, Lucey said.