A new long-term observational study is reporting a link between short sleep durations of under six hours in midlife and an increased risk of dementia in old age. The research cannot offer evidence of a causal link but with a 25-year follow-up period it is one of the longest studies to detect this association.
A large body of research linking dementia with poor sleep has accumulated over recent years. Much of this work has focused on how increasing sleep disturbances in old age can be an early indicator for cognitive decline and Alzheimer’s disease.
However, most research on the subject involves studies with short follow-up periods, often less than 10 years. One of the big challenges scientists face in dementia research is the growing understanding that these neurodegenerative diseases can slowly progress over decades before symptoms of cognitive decline ultimately appear.
Longitudinal studies tracking health over decades are deeply challenging, particularly when trying to tease apart cause and effect. This new study is by no means conclusive but it does offer novel insights into the potential relationship between shorter sleep patterns in middle-age and later-life dementia.
The researchers looked at data from a long-term project called the Whitehall II cohort study. Beginning in the mid-1980s, this study recruited around 10,000 subjects aged between 35 and 55. With a mean follow-up period of just under 25 years the researchers were able to correlate incidences of later-life dementia with sleep durations around the age of 50.
The study found late-life dementia risk increased by 30 percent in people with persistent short sleep durations across their 50s and 60s of less than six hours a night, as opposed to a normal seven hours. Importantly, this association was independent of any other demographic or social factors, including mental health issues.
The big question this research is unable to answer is whether these midlife sleep disturbances directly contribute to the development of dementia, or whether poor sleep is a very early symptom of the neurodegeneration that leads to dementia. Elizabeth Coulthard, from the University of Bristol, says these findings certainly build on research suggesting poor sleep plays a causal role in the onset of dementia.
“This study adds new information to the emerging picture because sleep is reported in a middle-aged cohort who are then followed over 30 years,” says Coulthard, who did not work on this new study. “This means that at least some of the people who went on to develop dementia probably did not already have it at the start of the study when their sleep was first assessed. So, it strengthens the evidence that poor sleep in middle age could cause or worsen dementia in later life.”
A number of recent studies have illustrated how poor sleep can contribute to diseases such as Alzheimer’s. Sleep has been found to play a vital role in clearing toxic proteins from the brain, so persistent poor sleep could play a causal role in the onset of cognitive decline. Robert Howard, from University College London, notes it is just as possible poor sleep is a very early symptom of disease.
“It’s always difficult to know what to conclude from these kinds of studies,” says Howard. “We know that the first signs of Alzheimer’s disease appear in the brain 20 years before detectable cognitive impairment, so it is always possible that poor sleep might be a very early symptom of the condition, rather than a treatable risk factor.”
In the new study, the researchers make clear the causal direction of this relationship is still subject to debate. The unique nature of this long follow-up study certainly offers insights into the association between short sleep and dementia but it is not known whether active measures to improve sleep patterns in middle-age can explicitly reduce one’s later life risk of developing dementia.
“While incipient dementia is known to affect sleep–wake cycles, the extent to which sleep duration over the adult lifecourse is associated with late-onset dementia is unclear because most studies have not explicitly considered age at assessment of sleep duration or the length of follow-up,” the new study concludes. “Public health messages to encourage good sleep hygiene may be particularly important for people at a higher risk of dementia.”
The new study was published in the journal Nature Communications.