Physiology Friday #110: 7 Hours of Sleep is the "Sweet Spot" for Brain Health and Structure

A new study finds that sleeping more or less than 7 hours per night is associated with poorer cognitive performance, mental health, and structural changes to the brain.

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I have a love-hate relationship with our culture’s “recent” fixation on sleep. 

On one end, it’s fantastic that everyone seems to be so obsessed with getting sufficient sleep and finding ways to promote sleep quality. Sleep is one of the pillars of health, and our societal shift away from glorifying the “no sleep grind” is a step in the right direction. 

On the other hand, sometimes I find it silly that we have to tell people to prioritize something as biologically fundamental as sleep. Like drinking water or eating food, sufficient sleep is a necessity and shouldn’t (emphasis on shouldn’t) be something we actively need to be reminded of. Alas, like many other health habits, nudges, prescriptions, and reminders can help us all make better decisions.

It is well-known that not getting enough sleep increases one’s risk for a variety of diseases and early death. Interestingly enough, observational studies have also found that long sleep duration is associated with negative health outcomes. I.e. — sleep duration exhibits a U-shaped curve with disease.

A “disease” of particular interest to sleep researchers is cognitive decline — encompassing dementia, Alzheimer's disease, and even milder forms of memory loss or cognitive impairment. Sleep serves the vital process of clearing out the brain, removing toxins, and promoting the formation of memories — explaining why a lack of sleep may predispose to poor brain health.

Unfortunately, previous data have only told us about the associations between sleep duration and indices of brain health and performance. What may be more useful to enhance our knowledge of the interaction between sleep, brain health, and aging is genetic data and brain imaging data that could provide mechanistic insight into how sleep changes the brain.

In a new study published in the journal Nature Aging, researchers analyzed sleep data, mental health and cognitive function assessments, and brain neuroimaging data from a cohort of 498,277 middle-aged to older adults to answer the following questions:

1. Is there a U-shaped relationship between sleep duration and mental health conditions and cognitive performance?

2. What is the relationship between sleep duration and brain structure?

3. What is the relationship and interaction between sleep, genetic risk score, brain structure, mental health, and cognitive function?

Findings

A sleep duration of 7 hours was associated with optimal cognitive function: A U-shaped relationship was found for fluid intelligence, numeric memory, pair matching, reaction time, and trail making — sleep durations below and above 7 hours were associated with inferior performance on these measures.

Sleep duration exhibited a U-shaped association with mental health measures: Sleeping more or less than 7 hours was associated with greater symptoms of anxiety, depression, mania, mental distress, self-harm, trauma, and reduced well-being.

Insufficient and excessive sleep durations were associated with adverse brain structural changes: Sleep durations below and above 7 hours per night were associated with reduced brain volume, gray matter area, and cortical thickness — particularly in the precentral gyrus, lateral orbitofrontal cortex, left insula, and the hippocampus.

Sleep variability was associated with poorer brain health: A larger difference in sleep duration from baseline to follow-up visit was associated with inferior cognition and mental health while maintaining sleep duration (i.e. a 0-hour change from baseline to follow-up) was associated with preserved cognitive health and function.

Insufficient and excessive sleep duration is most harmful to adults aged 44-59: The strongest U-shaped association between sleep, mental health, cognitive function, and brain volume was found in this group, with curves flattening as age increased (i.e. a weaker association in older groups).

Significant interactions were found between genetic risk, sleep, and brain health: Polygenic risk scores (PRSs) were found to significantly mediate the interplay between sleep duration, mental health, cognitive function, and brain structure.

Thoughts and musings

The non-linear associations uncovered here are not new. If you don’t sleep enough, you’ll suffer the consequences. If you sleep too much, that’s also not a good thing. 

Why long sleep duration is associated with worse brain health is a question that we don’t quite know the answer to, but one explanation is that individuals with “excessive sleep” may have afflictions that predispose them to sleep a lot. Another explanation could be that individuals might compensate for poor sleep quality with increased sleep duration. Again — somewhat speculative.

“How much sleep should I get?”

Based on these data — 7 hours is the sweet spot. The “magical” 8 hours gets thrown around as the optimal target, and I think this is actually a good “time in bed” goal to shoot for. Aim for 8 hours in bed and you’ll likely get 7 hours of sleep. In reality, somewhere between 7-9 hours per night is likely a sleep duration that works for most people and is what public health organizations like the National Sleep Foundation recommend for optimal health.

Our brain is the organ we must take the best care of, especially with age. You can exercise your heart, and I guess you can exercise your brain, but the best way to ensure optimal cognitive function and healthy brain aging is to prioritize sleep.

This study shows that non-optimal sleep structurally changes the brain! These structural changes may be early markers for cognitive decline and mental health disorders that appear later on in life.

If you can’t get 7 hours of sleep every night, there is no reason to worry. Chronic insufficient sleep is the risk factor that is most concerning — not the occasional night of sleep deprivation (though this can also have negative effects on performance!)

I don’t need to be the one to tell you how much better it feels to be well-rested compared to sleep-deprived, but it’s always nice to have data to back up the recommendations we hear on a daily basis. Treat your brain right and get a solid 7 hours tonight.

Thanks for reading. See you next Friday.

~Brady~

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Study cited

Li, Y., Sahakian, B.J., Kang, J. et al. The brain structure and genetic mechanisms underlying the nonlinear association between sleep duration, cognition and mental health. Nat Aging (2022). https://doi.org/10.1038/s43587-022-00210-2