Higher physical activity levels, less sedentary time, and 7 to 8 hours of sleep per night were associated with a lower risk of incident dementia in a large systematic review and meta-analysis of prospective cohort studies, although substantial variability across studies limits interpretation.

Researchers analyzed data from community-dwelling adults aged 35 years and older, including 49 studies of physical activity (nearly 2.9 million participants), 17 studies of sleep (more than 1.3 million participants), and 3 studies of sedentary behavior (nearly 296,000 participants). Across studies, about 3% of participants developed dementia over follow-up periods of approximately 7 to 12 years.

Physical activity at or approximating recommended levels—150 minutes or more per week—was associated with a 25% lower risk of dementia. However, heterogeneity across studies was very high (I² = 92.5%), indicating substantial variability in effect estimates.

Notably, subgroup analysis showed that this association was attenuated and not statistically significant among patients aged 65 years and older with follow-up of 10 years or more, suggesting the potential benefit of physical activity may be more pronounced earlier in adulthood.

Sedentary behavior of 8 hours or more per day was associated with a 27% higher risk of dementia. Heterogeneity was low (I² = 0%), but this finding was based on only three studies, two of which had serious risk of bias, limiting confidence in the estimate.

Sleep duration showed a nonlinear association. Sleeping less than 7 hours per night was associated with an 18% higher risk of dementia, and sleeping more than 8 hours was associated with a 28% higher risk, both compared with 7 to 8 hours per night.

The researchers emphasized that moderate-to-serious risk of bias across included studies and the observational design preclude causal inference; findings should be interpreted as associations rather than evidence of cause-and-effect relationships.

Proposed mechanisms may support biological plausibility. Physical activity has been linked to increased brain-derived neurotrophic factor and improved vascular function, while sleep may influence glymphatic clearance and neuroinflammation, processes implicated in neurodegeneration.

Additional limitations included reliance on self-reported physical activity and sleep, potential residual confounding, and the possibility of reverse causation, particularly in studies with shorter follow-up.

“Regular physical activity, less sedentary time and appropriate nightly sleep (7–8 h) may be associated with reduced risk of dementia and are potentially modifiable factors in the prevention or delay of dementia,” wrote lead study author Akinkunle Oye-Somefun of York University in Canada, and colleagues.

The researchers reported no conflicts of interest.

Source: PLOS One