Edgar Allan Poe may have loathed “those little slices of death”, but sleep is even more crucial to us than food. We can survive weeks without food — no more than a week without sleep (the record is 11 days). There’s a reason sleep deprivation is classed as torture.
But, like all good things in life, moderation is the key. Recent research indicates that both too little and too much sleep may increase cognitive decline.
Researchers studying Alzheimer’s disease, aware that poor sleep also contributes to cognitive decline, wanted to tease out the separate effects of sleep and Alzheimer’s on cognition. So they tracked cognitive function in a large group of older adults over several years. Cognitive function was analysed against levels of Alzheimer’s-related proteins and measures of brain activity during sleep.
Like porridge, chairs and beds, the trick is getting it “just right”.
“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 first author Brendan Lucey, MD, an associate professor of neurology and director of the Washington University 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.”
While others studies have indicated that self-reported short and long sleepers are more likely to perform poorly on cognitive tests, the possible effects of Alzheimer’s were not included in these studies.
In the newer study, volunteers underwent annual clinical and cognitive assessments, as well as providing blood and cerebrospinal fluid samples to measure levels of Alzheimer’s proteins. They also slept with a small electroencephalogram (EEG) monitor strapped to their foreheads for four to six nights to measure brain activity during sleep.
The researchers found a U-shaped relationship between sleep and cognitive decline. Overall, cognitive scores declined for the groups that slept less than 4.5 or more than 6.5 hours per night — as measured by EEG — while scores stayed stable for those in the middle of the range. EEG tends to yield estimates of sleep time that are about an hour shorter than self-reported sleep time, so the findings correspond to 5.5 to 7.5 hours of self-reported sleep, Lucey said.
The U-shaped relationship held true for measures of specific sleep phases, including rapid-eye movement (REM), or dreaming, sleep; and non-REM sleep. Moreover, the relationship held even after adjusting for factors that can affect both sleep and cognition, such as age, sex, levels of Alzheimer’s proteins
Still, each person’s sleep needs are unique. No-one needs to panic and change their sleep schedule, long or short, so long as they wake up feeling rested. But poor sleep can be a problem, which can be remedied. Sleep quality, rather than the specific quantity, is probably most important.
“Physicians who are seeing patients with cognitive complaints should ask them about their quality of sleep. This is potentially a modifiable factor […] Often patients report that they’re not sleeping well. Often once their sleep issues are treated, they may have improvements in cognition.”
Science Daily
So, as Samuel L. Jackson would say, “Go the f- to sleep…”
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