Sleeping an Hour Longer in Later Life Linked to Doubled Dementia Risk

The work is an observational study in people. It followed 3,057 participants from a Swedish population study called Good Aging in Skåne. Their average age at the start was 68.4. All of them slept a normal 6 to 8 hours a night at baseline. They were followed for up to 20 years, with an average of 8.39 years.

Observational means the researchers tracked what happened. They did not change anyone's sleep. So the study can show an association. It cannot show that longer sleep causes dementia.

The team, led by Overton, Elmståhl and Basna, looked at how nightly sleep changed between visits. People reported their own sleep. A change counted as prolonged if they gained more than one hour since the previous visit. It counted as shortened if they lost more than one hour. The researchers also measured variability: how much a person's sleep length swung from visit to visit.

Prolonged sleep stood out. According to the study, it was linked to significant cognitive decline, measured with a combined score of global thinking ability. This held after adjusting for health-related factors, including time spent in bed without sleeping.

The dementia figures were larger. Compared with people whose sleep stayed stable, those with prolonged sleep had a hazard ratio of 2.31 for dementia. The authors describe this as a 131% increased risk (p < 0.001). A hazard ratio compares how often an outcome occurs over time in one group against another.

Greater variability in sleep length was also linked to steeper cognitive decline. It was tied to a higher dementia risk too, with a hazard ratio of 1.49 (p < 0.001).

Shortened sleep was different. The study did not find a significant link between losing more than an hour of sleep and either cognitive decline or dementia.

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What does this mean for personal health? The authors say changes in sleep duration, especially prolonged sleep and high variability, may serve as markers of future cognitive impairment. A marker is a warning sign, not a cause. Sleeping longer may reflect changes already under way in the brain, rather than something that drives them.

Details it does not give, such as who dropped out, how dementia was diagnosed, or what other factors were adjusted for, could not be checked. The sleep data were self-reported, and the findings come from one Swedish group that began with normal sleep.

The practical reading is cautious. The authors propose that prolonged sleep and widely swinging sleep may be markers of future cognitive impairment, but the study offers no guidance for individuals on tracking their own sleep. It does not show that cutting back on sleep would lower risk. It does not test any fix.

What to watch next

Whether other long-term studies confirm the pattern, ideally with objective sleep measures, and whether changes in sleep can help flag people for earlier cognitive checks.

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