Adults who get six hours or less of sleep per night in their 50s and 60s are more likely than those who get at least seven hours of sleep to develop dementia in their 70s, according to a large new study in Nature Communications.
For the study, Séverine Sabia, an epidemiologist at the French public health research center Inserm, and colleagues assessed survey data collected via the University College London's Whitehall II study. That study, which started in 1985, monitored the health and lifestyles of more than 10,000 British civil servants.
For the latest study, researchers focused on 7,959 study participants, starting at about 50 years old, who self-reported their sleeping habits in reports filed six times between 1985 and 2016. According to the researchers, nearly half of those participants also had their sleep duration measured by accelerometers, generally around the age of 69, and the resulting data aligned with their self-reported figures.
During 25 years of follow-up, 521 study participants were diagnosed with dementia, at an average age of 77. The researchers found that participants who got six hours or less of sleep per night in their 50s and 60s appeared to be 30% more likely than those who got a minimum of seven hours of sleep to be diagnosed with dementia.
According to the researchers, the correlation held when taking into account factors such as education, marital status, physical activity level, cardiovascular health, and more. The correlation also held when researchers separated out participants who had a mental health diagnosis before the age of 65, since such conditions are often linked with sleep disturbances. The correlation also held whether or not participants took sleeping medication or had the ApoE4 mutation, which makes people more likely to develop dementia.
The researchers said that while their study identified only a correlation, not a causation, the findings suggest that "short sleep duration in midlife is associated with the higher risk of dementia later in life, independently of sociodemographic, behavioral, cardiometabolic, and mental health factors."
However, the researchers cited several limitations to the study, including how much of the sleep data was self-reported and that the quantitative measure—the accelerometer data—was collected later in the study, when participants were nearing 70, making it less helpful than it would have been if collected when participants were younger.
In addition, the researchers noted that nearly all participants were white, with higher degrees of education and better overall health than the British population at large. Further, they noted that they relied on EHR data to determine dementia diagnoses, which meant that some cases may have been missed and the various types of dementia could not be identified.
While experts praised the study, many cautioned that it does not resolve the ongoing debate about whether sleep issues cause dementia, or whether the pathology for dementia results in sleep issues. According to experts, that's especially true given that research in this field tends to yield confusing findings, including studies that have linked longer sleep patterns—nine hours or more—to a higher risk of dementia.
"It's always very difficult to know what to conclude from these kinds of studies," said Robert Howard, a professor of old age psychiatry at University College London, who submitted comments on the study to Nature Communications. "Insomniacs—who probably don't need something else to ruminate about in bed—shouldn't worry that they are heading for dementia unless they get off to sleep immediately."
For instance, he explained that because "the first signs of Alzheimer's disease appear in the brain 20 years before detectable cognitive impairment … it is always possible that poor sleep might be a very early symptom of the condition, rather than a treatable risk factor."
On the other hand, Erik Musiek—a neurologist and co-director of the Center on Biological Rhythms and Sleep at Washington University in St. Louis, who was not involved in the study—said the study does shed some light on the "chicken or egg" debate between poor sleep and dementia because it involved comparatively younger people over a long time frame.
"I don't know that this study necessarily seals the deal, but it gets closer because it has a lot of people who were relatively young," he said. "There's a decent chance that they are capturing people in middle age before they have Alzheimer's disease pathology or plaques and tangles in their brain."
That said, he added that while the study "provides a pretty strong piece of evidence that sleep is important in middle age … we still have a lot to learn about that and how the relationship actually occurs in people and what to do about it."
Kristine Yaffe, a professor of neurology and psychiatry at the University of California-San Francisco, who was not involved in the study, voiced similar thoughts. "It would be really unlikely that almost three decades earlier, this sleep was a symptom of dementia, so it's a great study in providing strong evidence that sleep is a really a risk factor," she said.
However, Yaffe also noted that there could be a "shared genetic link" between poor sleep and increased Alzheimer's risk, or that the sleep-dementia relationship is "bidirectional," with poor sleep increasing the odds of dementia, which in turn fuels more poor sleep.
Regardless, Yaffe and other experts said people aiming to get a good night's sleep should avoid sleeping pills—which "don't give you as deep of a sleep"—and focus instead on having a routine sleep schedule, avoiding caffeine before bedtime, and limiting technology use in the bedroom (Belluck, New York Times, 4/20; Sample, The Guardian, 4/21; Coleman, The Hill, 4/20).