How to find the ‘sweet spot’ between too little and too much sleep

By Joe Morgan

Sleep is best at preserving the brain when people find the 'sweet spot' between too little and too much sleep, according to a new study.

Both short and long sleepers experience greater cognitive decline than those who slept a moderate amount. Even when the effects of early Alzheimer's disease were taken into account, according to new research.

Poor sleep and Alzheimer’s disease are both associated with cognitive decline.

In the study, older adults were asked to provide samples of cerebrospinal fluid to measure levels of Alzheimer's proteins and each slept with a tiny monitor strapped to their foreheads for four to six nights to measure brain activity during sleep for an average of four-and-a-half years.

The majority had no cognitive impairments with the average age being 75 at the time of the study.

Cognitive scores declined for the groups that slept less than four and a half or more than six and a half hours per night.

The monitor tends to yield estimates of sleep time that are about an hour shorter than self-reported sleep time, so the findings correspond to five and a half and seven and a half hours self-reported shuteye.

Dr. Brendan Lucey, associate professor of neurology at Washington University Sleep Medicine Centre, said: “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.

“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."

"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? We need more longitudinal data to answer this question.”

Professor Beau Ances, at the Barnes-Jewish Hospital, said: “I ask many of my patients, ‘How’s your sleep?’

“Often patients report that they’re not sleeping well. Often once their sleep issues are treated, they may have improvements in cognition.

"Physicians who are seeing patients with cognitive complaints should ask them about their quality of sleep. This is potentially a modifiable factor.”

The findings were published in the journal Brain.