Anyone who has ever pulled an all-nighter can attest to how beneficial sleep can be when it comes to function throughout the day. Sleep deprivation can leave you feeling horrible and unable to concentrate on the task at hand.
Despite the mystery surrounding sleep, researchers have long understood that sleep is crucial to immune function, metabolism, memory, learning, and other bodily functions. In general, researchers have suggested that individuals should sleep for at least eight hours per night to function optimally.
However, a recent study published in the journal Brain from the Washington University School of Medicine suggests that sleeping too much can also have negative effects.
At Washington University School of Medicine, the research team was eager to understand how much sleep was associated with cognitive impairment over time. As both poor sleep and Alzheimer’s disease are associated with cognitive decline, researchers selected older adults for their sample. Moreover, the study tracked the cognitive function of 100 participants (aged mid-to-late-70s) for a period of four to five years.
Alzheimer’s is the most common cause of cognitive decline among older people, accounting for 70% of cases of dementia. Furthermore, the disease can also worsen when sleep is insufficient, as poor sleep is a common symptom of the disease.
Although separating out the effects of poor sleep and Alzheimer’s can be challenging, using this sample made the task more manageable. Additionally, as reported on SciTechDaily, “By tracking cognitive function in a large group of older adults over several years and analyzing it against levels of Alzheimer’s-related proteins and measures of brain activity during sleep, the researchers generated crucial data that help untangle the complicated relationship among sleep, Alzheimer’s, and cognitive function.”
At the beginning of the study, 88 participants showed no signs of dementia, while 12 participants displayed signs of cognitive impairment. Additionally, one of the 12 people with cognitive impairment had mild dementia, while the other 11 had pre-dementia. Although only a few participants showed signs of dementia, all were asked to complete both cognitive and neuropsychological tests that carefully separated the effects of sleep from Alzheimer’s disease.
In this study, participants were asked to give blood samples for testing for the high-risk Alzheimer’s genetic variant APOE4. In addition, they also provided samples of cerebrospinal fluid to measure Alzheimer’s protein levels. A tiny EEG monitor was also attached to the participants’ foreheads for four or six nights to measure brain activity while they slept.
When scientists analyzed all the data, they discovered that cognitive scores declined for groups of participants who slept less than 4.5 or more than 6.5 hours per night. Interestingly, the results point to how total sleep time may not be as crucial as sleep quality.
Researchers Believe “Sleep Quality” Could Be Key To A Good Night’s Rest
Associate professor of neurology and director of the Washington University Sleep Medicine Center Brendan Lucey wrote, “Our study suggests that there is a middle range, or ‘sweet spot,’ for total sleep time where cognitive performance was stable over time.” Researchers observed a U-shaped relationship between sleep and cognitive decline.
Even when specific sleep phases, such as rapid-eye-movement, dreams, and non-REM sleep, were taken into account, the U-shaped relationship remained unchanged. Furthermore, the relation held up when adjusting for factors such as age, sex, the presence of Alzheimer’s protein, and the presence of APOE4.
As measured by the EEG, participants who slept more than 6.5 hours per night or less than 4.5 hours per night were more likely to show cognitive decline. However, as Dr. Lucey suggested, participants whose scores fell within the middle range stayed stable, suggesting there is a “sweet spot.”
Co-senior author and professor of neurology David Holtzman, MD of the study explains, “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.” Dr. Holtzman also stated, “It suggests that sleep quality may be key, as opposed to simply total sleep.”
Dr. Lucey echoes those sentiments claiming, “Short and long sleep times were associated with worse cognitive performance, perhaps due to insufficient sleep or poor sleep quality.” Nevertheless, even with these compelling results, Dr. Lucey acknowledges that the fascinating link between sleep and cognitive functioning remains at best murky.
“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?” Dr. Lucey asks. To gain a better understanding of this question, researchers need to collect more longitudinal data or repeat observations over a specific period of time.
In the meantime, Dr. Lucey recommends that individuals continue with either their short or long sleep habits, as long as they feel rested.