One of the Biggest Sleep Studies to Date Just Uncovered Something Surprising About Your Brain and Body

It makes sense that your sleep quality can have a big impact on how you feel the next day (and often, the day after too, if you try to play sleep catch-up). But new research from Prenuvo, shared exclusively with Women’s Health, suggests how well you sleep may also have far-reaching implications for your brain and body composition.

The large study analyzed brain and metabolic biomarker information gathered during MRI scans. From there, researchers were able to tease out some interesting findings on how your sleep efficiency—or, the amount of time you spend in bed asleep—may impact your brain and body fat.

“Good sleep” is about more than just hours logged in bed, says Alex Exuzides, PhD, study co-author and vice president of research and medical affairs at Prenuvo. Here's what the study uncovered, plus what healthcare providers want people to take away from it.

Meet the experts: Alex Exuzides, PhD, study co-author and vice president of research and medical affairs at Prenuvo; Christopher Winter, MD, sleep specialist at Charlottesville Neurology and Sleep Medicine and author of The Sleep Solution: Why Your Sleep Is Broken and How to Fix It; Mir Ali, MD, medical director of MemorialCare Surgical Weight Loss Center at Orange Coast Medical Center in Fountain Valley, California; Thomas Michael Kilkenny, DO, director of the Institute of Sleep Medicine at Northwell's Staten Island University Hospital

What did the study find?

The study, which was presented at the Congress of the European Sleep Research Society, analyzed data from 14,031 adults who underwent whole-body MRI scans at Prenuvo. Participants also answered questions about how long and how well they slept over the previous month using the Pittsburgh Sleep Quality Index (PSQI)-based questionnaire.

The researchers discovered a lot about sleep's potential impact on the body:

• Higher sleep efficiency was linked with more parietal gray matter volume, which plays a role in working memory and attention.
• Sleeping for longer periods of time was linked with smaller hippocampal volume, which is associated with memory formation and emotional regulation. (The researchers noted that the association became weaker when it included people who slept more than eight hours and shouldn't be interpreted as "sleeping too much shrinks your brain.")
• People who reported sleeping more than eight hours had more visceral fat, i.e. fat that's stored deep in the abdomen.
• A higher sleep efficiency was linked with lower visceral fat levels.

The study was observational, making it impossible to determine whether sleep patterns or quality caused these differences. But the results still raise some important questions.

Why might poor sleep be linked to higher visceral fat?

This latest data supports what doctors and scientists already know. "There is ample evidence that poor sleep equals more visceral fat," says Christopher Winter, MD, sleep specialist at Charlottesville Neurology and Sleep Medicine and author of The Sleep Solution: Why Your Sleep Is Broken and How to Fix It. Higher levels of visceral fat are linked to cardiovascular disease, chronic inflammation, and more health issues, so you want to minimize it if you can.

When someone has poor sleep, levels of the stress hormone cortisol rise in their body, says Mir Ali, MD, medical director of MemorialCare Surgical Weight Loss Center at Orange Coast Medical Center in Fountain Valley, California. These people may also experience more insulin resistance, along with less movement and exercise, Dr. Winter says. "That can lead to more deposition of fat in the visceral area," Dr. Ali says.

There is a sweet spot here, though. "Too little or too much sleep can lead to visceral fat production," says Thomas Michael Kilkenny, DO, director of the Institute of Sleep Medicine at Northwell's Staten Island University Hospital.

Why might greater sleep efficiency be associated with greater gray matter volume?

That's not entirely clear. "Previous research has linked poorer, more disrupted sleep with smaller brain volume in areas including the parietal cortex," Exuzides says. "While our findings align with this research, we can’t determine why this relationship exists or whether better sleep leads to changes in brain structure."

Why is longer sleep linked with smaller hippocampus volume?

That mechanism is also not clear, but it's not the first time this has surfaced. "The [smaller] hippocampal volume was also mentioned recently in a 2023 UK Biobank study," Dr. Winter says.

Still, Exuzides says this can be tricky to parse out. "Reporting longer sleep doesn’t necessarily mean someone is sleeping longer or better," he says. "It may reflect time spent in bed—reading, scrolling through social media—disrupted sleep, underlying health conditions, or medication use."

How can you improve your sleep efficiency?

Dr. Kilkenny says that first, it’s important to separate those who don’t get much sleep because that’s all the time they have (habitual short sleepers) from those with insomnia. “Habitual short sleepers simply do not allow enough time to sleep. In these patients, if the sleep time was increased, they would be allowed to sleep,” whereas someone with insomnia wouldn’t be able to sleep longer no matter how much more time they had.

For habitual short sleepers, Dr. Kilkenny recommends adding more time in bed, particularly by going to bed earlier. For those with insomnia, Dr. Kilkenny says cognitive behavioral therapy may help, and is actually recommended by the American Academy of Sleep Medicine. And while it’s no long-term substitute for seeing an actual provider, Dr. Kilkenny says the free app CBT-i Coach “can be utilized as a starting point to assist in correcting insomnia.”

What's the takeaway?

Exuzides stresses that more research is needed to see how the amount and efficiency of your sleep impacts your health. But doctors say the findings are a good reminder that sleep can have big implications for your overall health. If you needed a reminder to practice good sleep hygiene and prioritize restful sleep as part of a well-balanced life, this might be it.