A new study has warned that sleeping for too long can have surprising effects on your brain
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Updated 17:21, 14 Mar 2025
Woman sleeping in bed
Too much - and too little - sleep can be bad for the brain, new research suggests
(Image: Getty Images/iStockphoto)
Many of us aren't getting enough sleep. This World Sleep Day (March 14), experts are championing the benefits of a good night's kip, while highlighting the medical, educational and social burdens caused by poor sleep.
However, a new study has warned that sleeping for too long could cause just as much harm as not sleeping long enough – at least when it comes to brain health.
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New research conducted by a team of researchers in Australia found that sleep is a "really important" predictor of brain health, and that both too much and too little shut-eye could be linked to diseases like dementia.
The study looked at more than 100 sleep studies of a total of around 110,000 people. Nearly three quarters of the studies (73 per cent) found that poor sleep and sleep disorders were linked to lower brain volumes, leading researchers to suggest that inadequate sleep could cause the gradual loss of nerve cells in the brain, called neurodegeneration.
Neurodegeneration can lead to a decline in brain function, as nerve cells in the brain stop working properly. This can cause a variety of symptoms, such as memory changes, difficulty moving, or problems with decision-making. It is also behind the memory loss seen with diseases like Alzheimer's disease and other forms of dementia.
MRI Scan of human brain
The study found that poor sleep was linked to lower brain volume(Image: Getty Images)
“We now have stronger evidence that sleep could be a really important factor when it comes to brain health,” said author of the study, PhD candidate Dr Tergel Namsrai at The Australian National University (ANU).
“We found, for example, people living with REM sleep behaviour disorders had lower grey matter volume in an area of the frontal lobe called the right frontal gyrus. This region is one of the first parts of the brain where we see evidence of dementia and Alzheimer’s disease," she explained.
Surprisingly, the researchers found that getting too much sleep had the same negative effects on the brain as getting too little. The study didn't specify how long was too long, but a number of previous studies have had similar results.
Results from a large sleep study published in 2018 found that those who more or less sleep than the optimal amount performed worse in cognitive tasks than those who hit the sweet spot.
“We found the optimum amount of sleep to keep your brain performing its best is seven to eight hours every night. That corresponds to what the doctors will tell you need to keep your body in tip-top shape, as well," said the study's lead author Conor Wild, Owen Lab Research Associate.
"We also found that people that slept more than that amount were equally impaired as those who slept too little,” Wild added.
Woman struggling to sleep
Our sleep and the health of our brains are inextricably linked
(Image: Getty Images)
Similarly, a study published in 2017 found that sleeping for more than nine hours could be a marker of early neurodegeneration, and an early warning sign of dementia.
Other studies have also linked sleep disturbances to declining brain health, with research published in January revealing that difficulty reaching deep sleep could be an early sign of Alzheimer's disease.
Dr Namsrai said the results of the recent study highlight the importance of monitoring and managing sleep health. “It underscores the need to assess sleep during routine medical check-ups, and trying to catch complaints early on, before they progress to major sleep disorders. Early intervention is critical,” she said.
“It’s important any future research also includes diverse populations, covering different age ranges, professions and ethnic origins, while also accounting for things like shift work that are linked to poor sleep. This will allow for more targeted and personalised intervention,” Dr Namsrai.
Responding to the study on the r/science subreddit, one Reddit user wrote: "Interesting that long sleep is just as bad for the brain as short sleep. Gotta hit that sweet spot of 7-9 hours."
Another commented: "I can attest to this. I have a mild sleep apnea and my cognitive functions are slow and almost nonexistent. My brain works, it's just really slow or feels really foggy. That might answer some questions about what's considered low quality sleep, among other things."
A third joked: "Me, reading the article at 3am: "Fascinating.""
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The peer-reviewed study was published in the journal Sleep Medicine.