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Turns Out, Too Much Sleep Might Mess With Cognition

Eliza Sullivan
September 24, 2020
Eliza Sullivan
mbg Nutrition & Health Writer
By Eliza Sullivan
mbg Nutrition & Health Writer
Eliza Sullivan is a food writer and SEO editor at mindbodygreen. She writes about food, recipes, and nutrition—among other things. She studied journalism at Boston University.
Young white woman sleeping in bed
Image by Javier Díez / Stocksy
September 24, 2020

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Our sleep habits have a whole lot of power over our health, and we know that getting too little sleep can sometimes have harmful effects on the brain and beyond—but is there such a thing as getting too much sleep? According to a new study from a research team in Beijing, there might be. The peer-reviewed paper found that too few and too many hours in bed were linked to more rapid cognitive decline1 over time.

How scientists are studying the connection between sleep duration and cognition.

Using data from two long-term population studies on aging (the English Longitudinal Study of Ageing and the China Health and Retirement Longitudinal Study), the researchers behind this new paper pooled a group of over 28,000 individuals to establish the link between "extreme sleep patterns" and cognition over time.

The participants reported their nightly sleep duration and participated in a series of cognitive tests, including immediate and delayed recall tests and fluency tests.

Researchers found that over time, participants who slept less than four hours or more than 10 hours a night experienced more rapid mental decline than a control group, who were getting seven hours of sleep each night.

According to the paper, this suggests that "cognitive function should be monitored in individuals with insufficient (≤4 hours per night) or excessive (≥10 hours per night) sleep duration," though the team acknowledges that "future studies are needed to examine the mechanisms of the association between sleep duration and cognitive decline."

The exact mechanism by which sleep patterns affect cognition needs to be researched further, but the paper does refer to a 2016 study that found that getting more or less than seven hours of sleep was linked to the thinning of cortical region of the brain, even in adults with normal cognition.

This finding tracks with existing research on sleep and brain health.

This is by no means the first study to consider the link between sleep duration and cognition, and sleep patterns have previously been studied as potential markers of Alzheimer's disease.

Furthermore, the dangers of getting too much sleep have also been a topic of investigation in the past. Studies have shown that people who use that pesky snooze button are more likely to feel groggy in the mornings, likely due to a phenomenon called "sleep inertia," which causes that foggy feeling upon waking and sometimes makes even the simplest tasks feel challenging.

So how much sleep do you actually need?

The number we hear the most associated with sleep is eight hours, but does that really hold true? And if so—why did the researchers have the control group getting only seven hours of sleep?

Well, the amount of sleep we need is largely dependent on our age. This study included participants with an average age of approximately 61, which, according to the National Sleep Foundation (NSF), would mean they need anywhere from seven to nine hours of sleep per night.

Given the data from this new study, that range is still within the "safe" zone for preserving cognition—it's only the extremes of less than four or more than 10 that they call out as potentially damaging, though further specific study will be needed to confirm and explain these results.

So if you're curious about how much sleep you personally need each night, check out this detailed explainer on what the right amount of sleep is for different age groups.

Eliza Sullivan author page.
Eliza Sullivan
mbg Nutrition & Health Writer

Eliza Sullivan is an SEO Editor at mindbodygreen, where she writes about food, recipes, and nutrition—among other things. She received a B.S. in journalism and B.A. in english literature with honors from Boston University, and she has previously written for Boston Magazine,, and SUITCASE magazine.