A new analysis of longitudinal data from British adults revealed that those who sleep less than seven hours per night during middle age are at an increased risk of developing dementia once they reach 70 years old, highlighting the importance of good sleep habits throughout adulthood.

In a paper published Tuesday in Nature Communications, a group of researchers used data from a 30-year study of nearly 8,000 adults in the U.K. to investigate the link between sleep duration and incidence of dementia. Comparing unusual sleep patterns to what is seen as the normal, seven-hour nightly sleep duration, they reported that consistently sleeping for six hours or fewer per night at age 50, 60 and 70 was associated with a 30% increased risk of dementia "independently of sociodemographic, behavioral, cardiometabolic, and mental health factors."

 They found that consistently sleeping for six hours or fewer per night at age 50, 60 and 70, as compared with normal sleep durations, was associated with a 30% increased dementia risk independent of sociodemographic, behavioral, cardiometabolic and mental health factors. 

Séverine Sabia, a research associate at the University of Paris, Inserm and first author of the paper, explained to The Academic Times that despite pathological processes typically unfolding for 10-20 years before the clinical diagnosis of dementia, there have only been a few studies that have examined sleep duration long before dementia diagnoses. Most have followed participants for less than 10 years prior to diagnosis, and results have been inconsistent.

"We showed a consistent association between short sleep duration in midlife and risk of dementia," she said. "This association was not explained by mental disorders and other chronic conditions known to be associated with dementia. Furthermore, [our] findings were confirmed using an objective measure of sleep duration in a sub-sample," Sabia continued, referring to an associated experiment in which participants wore watch accelerometers for one week to track their sleep.

The authors noted in the paper that their findings cannot establish cause and effect, but can suggest that there is a strong connection between sleep duration and dementia risk. A common symptom of diagnosed cases of dementia is altered sleep, and plenty of prior studies have confirmed that time spent sleeping is linked to dementia risk in adults older than 65. But the association is understudied in younger age groups.

For their analysis, the researchers sourced data from University College London's Whitehall II study that has examined the health of 7,959 British adults since 1985. One component of the survey had participants self-report their sleep duration six times between 1985-2015. This allowed for an assessment of sleep duration for each participant at ages 50, 60 and 70 and a comparison of those who developed dementia with those who did not, up until 2019.

At the end of the follow-up period, there were 521 diagnosed cases of dementia among the nearly 8,000 adults in the study. Most cases were diagnosed when the individual was older than 70. The lowest dementia rate was observed among those who reported sleeping for seven hours per night, categorized as a normal duration, regardless of age. And short sleep duration, categorized as six hours or less per night, was associated with the highest risk of dementia, also regardless of age. There was no evidence of an association between dementia and long sleep duration, categorized as more than eight hours per night.

There was a 30% increased dementia risk in those with consistently short sleeping patterns between the ages of 50-70, irrespective of cardiometabolic issues, such as diabetes and hypertension, or mental health issues. Both kinds of issues are known risk factors for dementia. Participants consistently sleeping for seven hour per night at age 50 were more likely to be male, white and married and to have good cardiometabolic and mental health overall.

In the paper, the authors noted that the association between short sleep duration and dementia not being attributable to mental health was a key finding, given that depression and mood disorders are related to changes in sleep and have been believed to play an important part in sleep and dementia. But the researchers' analysis of depressive symptoms and central nervous system medications in the participants did not show mental health to explain the association.

"Some studies have also suggested that long sleep duration was associated with increased risk of dementia, but findings remain inconsistent, probably due to the small number of long sleepers in the study population," Sabia said. "In the present study, there was no robust evidence of an association with long sleep, but further studies including a higher number of long sleepers are required to examine the role of long sleep in dementia."

To confirm their conclusions, the researchers also analyzed results from a sub-study of the original Whitehall survey that was conducted in 2012-2013. It used a smaller sample of adults between 60-83 years old. They were asked to wear watch accelerometers overnight for one week in order to objectively track sleep duration and avoid any biases that may have been present with self-reporting their sleep. Sleep duration was estimated using a validated algorithm guided by a sleep log, and the results of the sub-study were consistent with the survey data results.

The connection between short sleep and dementia may be explained by several processes, the authors said, including neuroinflammation, which is the inflammation of the body's nervous tissue that is typically associated with neurodegenerative diseases; and impaired amyloid beta clearance. As Sabia explained, sleep plays an important role in the clearance of protein waste in the brain — when we are awake, neuronal activity increases the release of amyloid beta proteins, and these proteins are then washed away from the brain during sleep. 

If we consistently do not get enough sleep, the clearance of these proteins might be altered. This can lead to the accumulation of amyloid beta in the brain, which has been observed in people with Alzheimer's disease. Prior studies have also shown that sleep deprivation has a detrimental effect on cognitive performance and amyloid beta clearance, and that amyloid plaque build-up contributes to poor sleep in older adults, highlighting a potential feedback loop between sleep and dementia.

"Public health messages to encourage good sleep hygiene may be particularly important for people at a higher risk of dementia," the authors said in the paper, which was a collaboration between University College London and the University of Paris, Inserm, a public research institution in France.

Sabia plans to continue this research using follow-up data from the Whitehall accelerometer sub-study to examine whether circadian rhythm features, such as rhythm amplitude and robustness, sleep characteristics, physical activity and light exposure, have any effect on the risk of dementia.

The study, "Association of sleep duration in middle and old age with incidence of dementia," published April 20 in Nature Communications, was authored by Séverine Sabia and Archana Singh-Manoux, the University of Paris, Inserm, and University College London; Aurore Fayosse, Julien Dumurgier, Claire Paquet and Aline Dugravot, the University of Paris, Inserm; Vincent T. van Hees, Accelting of the Netherlands; Andrew Sommerlad, University College London and the NHS Foundation Trust; and Mika Kivimäki, University College London and the University of Helsinki.