Are you sleeping too much? New research reveals how many hours adults really need for brain health |
Most of us worry about not sleeping enough, the bleary mornings, the heavy eyelids, the sense of running on fumes. What almost no one considers is the opposite problem: the idea that getting too much sleep might quietly work against your brain. It sounds counterintuitive. It even borders on unfair. But researchers in Washington, US, found that the range of sleep linked to better brain health may be smaller than expected, with both short and long nights tied to decline, a finding that contradicts years of official guidance and could make even good sleepers pause.And yes, it’s unsettling. Most of us have had nights where eight or nine hours felt like a luxury; few imagine it could put them in the same risk bracket as someone sleeping far too little. But that’s exactly what one new study suggests.
What the Washington researchers found
Their study looked at a group of older adults whose sleep varied widely but whose sleep quality was consistently poor. The striking finding was that people sleeping less than 4.5 hours a night, and those sleeping more than 6.5 hours, showed greater risk of cognitive decline over time. The pattern, they noted, resembled the kind of deterioration associated with ageing ,one of the strongest predictors for conditions such as Alzheimer’s. They even suggested an ideal zone: between 4.5 and 6.5 hours of sleep, at least for the specific population they studied. It’s a narrow band, and the conclusion is the kind that makes both the public and the sleep-science community sit up a little straighter.
Why this challenges what health bodies say
Writing in The Conversation, senior psychology lecturer Greg Elder acknowledged that the results run counter to what major health bodies teach. He explained that, “The study showed that sleeping longer than 6.5 hours was associated with cognitive decline over time, this is low when we consider that older adults are recommended to get between seven and eight hours of sleep every night.” Elder notes that neither the NHS nor the US Centres for Disease Control (CDC) currently suggest that sleeping more than 6.5 hours is harmful. But he emphasises that the Washington study measured sleep quality as well, and that all participants who slept very little or a lot were also sleeping badly.
Why might sleep duration affect the brain?
He also notes that researchers still don’t fully understand why lack of sleep is linked to cognitive decline. “One theory is that sleep helps our brain flush out harmful proteins that build up during the day,” he writes. “So interfering with sleep might interfere with our brain’s ability to get rid of these. Experimental evidence even supports this, showing that even just one night of sleep deprivation temporarily increases beta-amyloid levels in the brain of healthy people.” Beta-amyloid is one of the proteins associated with Alzheimer’s disease. The theory is not a certainty, but Elder points to it because it illustrates how sensitive the brain may be to disrupted sleep.
So is it too much sleep, or something else entirely?
At the same time, he is careful about what the study cannot claim. All participants, regardless of how long they slept, struggled with poor sleep quality. That detail matters.Elder suggests that the real story may be less about the clock and more about what happens while you’re asleep. “It could be the case that it isn’t necessarily the length of the sleep that matters, but the quality of that sleep when it comes to risk of developing dementia,” he says.Although the study found a correlation between sleeping more than 6.5 hours and cognitive decline, Elder stresses an important point: the people sleeping the most may have had underlying issues that weren’t detected in the tests.“For example, this could include poor health, socioeconomic status or physical activity levels. All of these factors together may explain why longer sleep was linked to cognitive decline.” In other words, long sleep may be a marker of something else, not a cause in itself. It’s a reminder that correlation and causation rarely hold hands as neatly as we’d like.
Another study says sleep duration may be genetic
While the Washington findings challenge conventional assumptions, another group of researchers on the opposite side of the US is offering an even more complicated picture. A separate study from San Francisco centred on Familial Natural Short Sleep (FNSS), a genetic trait in which people naturally function fully on four to six hours of sleep. These individuals aren’t sleep-deprived; they simply don’t need the traditional eight-hour benchmark.Neurologist Louis Ptacek, lead author of the study, said: “There’s a dogma in the field that everyone needs eight hours of sleep, but our work to date confirms that the amount of sleep people need differs based on genetics. Think of it as analogous to height; there’s no perfect amount of height, each person is different. We’ve shown that the case is similar for sleep.” Ptacek and his team have spent over a decade studying these short sleepers, though he warns that understanding the genetics will take time. Mapping the sleep genes, he says, is like solving “a thousand piece jigsaw puzzle.” What they can say for certain is that some people are built for shorter nights, and that treating them as chronically underslept would be a mistake. The San Francisco researchers also point out how often sleep patterns intersect with neurological disease. “Sleep problems are common in all diseases of the brain,” they write, explaining that sleep relies on many parts of the brain functioning together. When any of these areas are damaged, disrupted or degenerating, sleep disturbances often follow.
What all of this actually means for sleepers
Taken together, the two studies paint a picture that is more nuanced than any single recommendation can capture. On one hand, sleeping significantly less than 4.5 hours or more than 6.5 hours in a population already experiencing poor sleep quality was associated with cognitive decline. On the other, the San Francisco team reminds us that some people are genetically wired to do perfectly well on short nights, and that sleep quantity is far from the whole story. For readers, it’s tempting to interpret the Washington findings as a directive, a new rule to replace the old one. But neither Elder nor the researchers themselves suggest rewriting national guidelines based on a study of 100 people. Instead, Elder leans toward a simpler, more intuitive takeaway: focus less on the number of hours you’re asleep, and more on how well your sleep restores you. If anything, the emerging research nudges us toward a more personalised understanding of sleep. The idea that everyone needs eight hours may have been a convenient shorthand, but the science, and the people studying it, are telling a more complex story. Sleep is individual. Sleep is layered. And sometimes, sleep is too much of a good thing, not because of the number on the clock, but because of what the brain is struggling with while you’re in it.
