7–8 Hours of Sleep Linked to Lowest Dementia Risk: Why Too Much Sleep Is Also Dangerous

Canadian researchers at York University have identified a target sleep duration associated with the lowest risk of cognitive impairment in old age. Combining the results of dozens of long-term studies involving more than 1.3 million people, the scientists found that those who regularly slept 7–8 hours a night had the lowest likelihood of developing dementia. At the same time, the statistical risk rose both with chronic sleep deprivation and with excessive time in bed.

The results of the large-scale work were published in a meta-analysis in the journal PLOS One. The authors analyzed data from 17 cohort studies of sleep with an average follow-up period of about 12 years. The link between sleep and neurodegeneration turned out to follow a pronounced U-shaped curve:

• sleeping less than 7 hours a night correlated with an 18% higher risk of dementia compared with the “normal” group;
• regularly sleeping more than 8 hours showed an even more alarming link — a 28% increase in risk.

The reverse-causality trap: why long sleep is a warning sign

Despite sensational headlines about “preventing Alzheimer’s with the right schedule,” the study’s authors urge people not to confuse statistical correlation with direct cause. The observational format of the meta-analysis does not prove that long sleep itself destroys brain cells.

Medicine has long recognized the phenomenon of reverse causality. The need to sleep 9–10 hours and constant daytime drowsiness are often not the culprit but an early hidden symptom of organic damage to the nervous system that has already begun. Pathological processes can damage the brain regions responsible for circadian rhythms for years before a person receives an official diagnosis. So artificially trying to sleep less than normal when impairments are already present is unlikely to protect the brain from the disease.

Exercise offsets a sedentary lifestyle

Besides sleep, the scientists assessed the impact of other everyday habits, according to the science news site ScienceAlert. Regular physical activity turned out to be one of the most reliable non-drug ways to support cognitive health — physically active participants had a 25% lower risk of dementia.

By contrast, a sedentary lifestyle (more than eight hours in a chair a day) was associated with a 27% higher likelihood of dementia. The authors note, however, that the data block on sedentary work rests on just three studies and requires further scientific verification.

The bottom line: the study confirms that a range of 7–8 hours of nightly rest remains the gold standard for adults. Sharp deviations from this schedule in either direction are a reason to take a closer look at your overall health and lifestyle.