Connecting the dots between sleep and cognition has long been tricky for researchers, mostly because the relationship is bidirectional. That is, poor sleep can trigger cognitive decline, but also, symptoms of that same decline can disrupt sleep. But, a new study teasing out this particular relationship in older adults offers some clear insight into just how much sleep per night may be ideal for brain health long-term.

The research clocked the sleep patterns for four to six nights of about 100 older people (with an average age of 75), all of whom had also undergone a battery of cognitive tests for up to 12 years prior. Because the study could uniquely consider a handful of cognitive- and sleep-related data points within a singular set of people at once, the researchers were able to uncover a happy medium for sleep duration when it comes to maintaining brain health: between 5.5 and 7.5 hours per night (though the exact amount will vary, depending on your age).

“There’s this inverse U-shaped relationship that we found between sleep and cognitive decline, where the people sleeping less than that amount or more than that amount were more likely to show a decrease in cognitive health over time,” says lead author on the study Brendan P. Lucey, MD, associate professor of neurology and director of the Washington University Sleep Medicine Center.

There’s a happy medium for sleep duration when it comes to maintaining your brain health: between 5.5 and 7.5 hours per night.

To parse this relationship, the researchers specifically chose participants for the sleep study who were already involved in Washington University’s longitudinal Alzheimer’s research program—meaning they already had, on average, 4.5 years of data for these folks on a host of relevant metrics, like biomarkers and genetic information related to Alzheimer’s risk, and scores on cognitive tests conducted each year to clinically measure things like memory and executive function.

“In this way, we had an overall picture of each participant’s change in cognition over time, and then could analyze that in relation to the amount of sleep they were getting during the sleep study,” says Dr. Lucey. That last bit was conducted by having the participants wear small electroencephalogram (EEG) machines strapped to their foreheads for several nights (while sleeping at home), in order to rule out the discrepancies that can come with people self-reporting their own sleep habits.

Once the researchers were equipped with all of the above longitudinal data, they were able to essentially control for any inherent symptoms of cognitive decline already in play at the time when the sleep study was conducted (that might’ve separately interfered with sleep), and figure out if there was an independent link between sleep duration and cognitive dips. With those factors out of the equation, the relationship still stood: Too muchor too little sleep was tied to a decrease in overall brain health.

Why sleep duration and brain health may be so closely linked

It may seem counterintuitive that if sleep is good for brain health, you can actually get too much of it. But the key point to note here is that quantity doesn’t always equal quality—and Dr. Lucey suspects that those participants getting more than 7.5 hours of sleep per night, on average, were also the ones experiencing lower quality sleep, hence the overall negative effect on their cognition over time.

“What may be happening in these cases is there’s something interfering with their sleep quality, like sleep apnea or restless leg syndrome, or perhaps a form of chronic insomnia, where maybe they’re being woken up throughout the night, but in total, they still get a lot of sleep,” says Dr. Lucey. “You can equate this kind of sleep to skipping along the surface, like a stone skipping on a pond: They’re getting under the water a little bit all night long, but they’re not getting to the deep, restorative stages of sleep that we need,” he says.

On the other end of the spectrum, those are the key parts of sleep that you’ll typically bypass if you’re not getting enough sleep. And that’s why Dr. Lucey emphasizes the importance of checking in with how you actually feel during the day, and not relying on the amount of time you spend in bed or asleep as the only box you need to check off for guaranteeing healthy sleep habits.

No matter your age or whether you have a known sleep-compromising condition, if you’re feeling unrested during the day, it could be a sign that something’s getting in the way of your ability to bank high-quality sleep. And this study’s findings on the link between sleep and brain health are all the more reason to address recurrent low-quality sleep with a health-care provider, adds Dr. Lucey, as there are a variety of interventions that can help correct it sooner than later—which, in this case, could make all the difference.

—reviewed by Smita Holden, MD

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