In this episode of the Better Science Show, Saleh and Fabio look at a large study about sleep and dementia. The study followed thousands of British adults for many years. It found a link between short sleep in middle age and a higher rate of dementia later in life. It did not prove that short sleep causes dementia.

What Did the Researchers Do?

The researchers used data from a long-running study of British civil servants. Their main analysis included 7,959 people with sleep information around age 50. Over an average of about 25 years, 521 people were diagnosed with dementia.

Most sleep reports came from a simple question: How many hours do you sleep on an average weeknight? The researchers grouped the answers as short sleep, normal sleep, or long sleep. Short sleep meant six hours or less. Seven hours was the comparison group. Long sleep meant eight hours or more.

A smaller group wore a device on the wrist that measured movement. This gave the team another way to estimate sleep time instead of relying only on memory.

What Did the Study Find?

Compared with people who reported seven hours of sleep, those who reported six hours or less had a 22% higher relative risk of dementia at age 50. At age 60, the relative risk was 37% higher. At age 70, the result was less clear because the range of likely values included no difference.

The team also looked at sleep patterns over time. People with a pattern of short sleep had about 30% higher relative risk than people with a pattern of seven-hour sleep.

These numbers describe differences between groups. They do not tell us that a given person will develop dementia. They also do not mean that short sleep raised the risk by 30 percentage points.

What Can This Study Not Tell Us?

This was an observational study. That means researchers watched what happened instead of assigning people to different amounts of sleep. The design can find a link, but it cannot prove that too little sleep caused dementia.

The researchers accounted for many measured differences, including age, smoking, physical activity, heart health, and mental health. Other differences may still help explain the result. Early brain changes can also affect sleep years before dementia is diagnosed, which makes cause and effect hard to separate.

The study had other limits. Most sleep data came from people’s own reports. Dementia diagnoses came from health records, which may miss some mild cases. The group was made up of civil servants and was healthier than the general population, so the exact results may not apply to everyone.

Why Might Sleep and Dementia Be Linked?

The study did not test a biological cause. The authors pointed to ideas from earlier research. Poor sleep may affect swelling in the brain, blood vessels, or the brain’s way of clearing waste. Dementia-related brain changes may also disturb sleep. More research is needed to understand what is happening.

The Bottom Line

This study adds useful evidence that short sleep in middle age is linked to dementia later in life. It does not show that sleeping longer will prevent dementia. Healthy sleep is still important for daily health. Anyone who often cannot sleep, or who is very sleepy during the day, should discuss it with a health professional.

Reference

Sabia S, Fayosse A, Dumurgier J, et al. Association of sleep duration in middle and old age with incidence of dementia. Nature Communications. 2021;12:2289.