PLOS One: Seven to Eight Hours of Sleep May Help Reduce Dementia Risk

The authors combined the results of 69 studies involving nearly 4.5 million people, including adults aged 35 and older. They examined the associations between dementia risk and three modifiable factors: physical activity, time spent sitting and sleep duration. The findings showed that sleeping fewer than seven hours a night was associated with an approximately 18% higher risk of dementia, while sleeping more than eight hours was associated with a 28% higher risk. A sedentary lifestyle — spending at least eight hours a day sitting — and low levels of physical activity, defined as less than 150 minutes a week, were also associated with increased risk.

“Regular physical activity, less time spent sitting and sufficient nighttime sleep — seven to eight hours — may be associated with a lower risk of dementia,” the researchers noted.

The authors emphasize, however, that these findings reflect statistical associations rather than a proven cause-and-effect relationship. In particular, excessive sleep may not cause dementia but could instead be an early sign of Alzheimer's disease already developing. At this stage, changes in the brain may not yet be externally noticeable but may already be affecting sleep patterns.

The value of the study lies in bringing together three everyday habits that people can control themselves and showing that each is independently associated with dementia risk. The authors also emphasize that their analysis included people aged 35 and older, whereas similar studies typically focus on older adults. This suggests that maintaining healthy sleep and physical activity habits may be worthwhile long before old age.

Further research is needed to establish causal relationships, but moderate physical activity and adequate sleep remain sensible recommendations for maintaining brain health.