People in their 50s and 60s who sleep for six or less hours each night are at increased risk of developing dementia, compared to those who consistently sleep for seven or more hours.

A major study by UCL and Inserm researchers found that people who regularly sleep for six hours or less were 30% more likely to develop dementia in older age. 

Researchers used data collected from 7,959 adults, over a period of 25 years. The participants reported their sleep duration six times between 1985 and 1988 (when they were aged 35 to 55 years). They then reported again between 2015 and 2016 (aged 63 to 86). Some volunteers wore watches which confirmed how long they slept. 

More than 500 participants had developed dementia by 2019, with most being in their late 70s. The association between dementia and poor sleep was made independent of sociodemographic, behavioural, cardiometabolic, and mental health risk factors.

Does lack of sleep cause dementia?

The study does not prove that lack of sleep causes dementia, as sleep impairment is also an early symptom of dementia. It concludes: “it remains unclear whether sleep duration prior to old age is associated with dementia incidence.” 

Author of the study, Dr Séverine Sabia told UCL News: “Sleep problems are known to occur in people with dementia, but it remains unclear whether sleep duration in midlife affects the risk of developing dementia at older ages. Here, by using a very long follow-up period, we have found that short duration sleep in midlife, assessed more than 25 years before mean age at dementia onset, is associated with dementia risk in late life.

“While we cannot confirm that not sleeping enough actually increases the risk of dementia, there are plenty of reasons why a good night’s sleep might be good for brain health. These findings confirm the importance of sleep hygiene for health.”

Why is sleep important for good brain health?

There is a wealth of research that shows how important sleep is to maintaining good brain health. For instance, lack of sleep affects cognitive function, making it more difficult to make decisions and concentrate. 

Quality of sleep is just as vital as how long you sleep. Studies have shown that men aged 65 plus who struggled to get to sleep or woke up a lot during the night were approximately 50% more likely to suffer cognitive decline.