Sleep duration linked to dementia risk

A study published in April 2026 in the journal PLOS One by epidemiologists Akinkunle Oye-Somefun and Chris Ardern from York University found that both sleeping too much or too little is associated with an increased risk of dementia.

Study spans large population and long time frame

The research aggregated data from multiple cohorts followed for over ten years, a volume rarely achieved in sleep research. The ideal sleep duration for adults was found to fall within a one-hour window, specifically between seven and eight hours per night.

Large-scale analysis of sleep and dementia

The study analyzed 17 cohort studies involving 1,344,170 adults, none of whom had been diagnosed with dementia at the start of the study. Participants were followed on average for 12.1 years and were 67.5 years old at inclusion.

Dementia risk varies with sleep duration

Nearly 50,000 participants received a dementia diagnosis during the follow-up period. Each study compared short and long sleepers to those who reported seven to eight hours of sleep. Sleeping less than seven hours was associated with an 18% higher risk of dementia, while exceeding eight hours increased the risk by 28%.

Interpretation of long sleepers remains unclear

The case of long sleepers remains difficult to interpret, as prolonged sleep could be an early sign of the disease rather than a triggering factor. The authors of the study note that the relationship is associative, without determining the direction of causality.

Sleep measurement method introduces uncertainty

The method of sleep measurement adds uncertainty, as the 17 studies were based on self-reported questionnaires rather than sensors. A person can spend nine hours in bed without recovering, and the quality of sleep was not considered in any of the cohorts.

Glymphatic system and sleep

During sleep, the glymphatic system, which clears brain waste, operates at full speed. A short night may limit this process. For long nights, the authors suggest that associated diseases or neurodegenerative changes may be involved.

Sedentary behavior also affects dementia risk

Sedentary behavior was found to have a comparable effect on dementia risk. Three studies measured the time spent sitting, which limits the scope of the findings. Beyond eight hours per day of sitting, the risk of dementia increased by 27%.

Physical activity lowers dementia risk

Conversely, adults who engaged in 150 minutes of moderate activity per week showed a 25% lower risk of dementia. The section on sedentary behavior covered 295,809 people, compared to nearly 2.9 million for physical activity.

Future research directions suggested

A follow-up starting as early as 35 years old is highlighted as a key area for future research. The authors suggest that future studies should use smartwatches rather than questionnaires to more accurately measure these factors.