Many studies suggest that lack of sleep may be associated with an increased risk of dementia. However, the relationship between poor sleep and dementia may be bidirectional, that is, poor sleep may increase one’s risk of dementia while increased levels of the misfolded proteins in the brain (such as amyloid plaques) that are present in dementia patients may negatively impact sleep quality. These misfolded proteins accumulate in the brain more than a decade before the onset of cognitive symptoms. Most of the previous research examining the relationship between sleep and dementia have only followed patients for less than ten years. However, a new study looked at whether poor sleep may be associated with dementia risk 25 years before the onset of symptoms .
This report, published in Nature Communications used data from 7,959 participants in the Whitehall II study, a cohort of British civil servants, and examined whether sleep duration is associated with dementia risk up to 25 years later . Individuals, aged 35-55, were enrolled between 1985-1988. Every 5-10 years they returned to the clinic and were asked how much they slept on an average weeknight. The investigators used public health records to assess whether an individual was diagnosed with dementia over the subsequent years.
They reported that short sleep duration (<6 hours per night) in individuals in their 50s or 60s was associated with a 22% and 37% increased risk of dementia, respectively, even after adjusting for other health behaviors, cardiovascular/metabolic, and mental health risk factors. Individuals in their 70s who reported short sleep, however, were not at an increased dementia risk.
Since the investigators collected data over such a long time, they also looked at sleep trajectories (e.g., did patients report short sleep earlier in life and normal sleep later in life). Persistent short sleep was associated with a 30% increased risk of dementia, while persistently long sleep or changes from short sleep to normal sleep were not associated with an increased risk.
One weakness in sleep epidemiological studies is that they often rely on self-reported measures of sleep duration. However, the investigators also used accelerometer data, a more objective measure, and found that there was a 63% increased risk of dementia in those with short sleep duration (1 hour 16 minutes to 6 hours 13 minutes) versus those who had ~7 hours of sleep. There was no increased risk with those who slept longer than 8 hours.
This study contradicts other studies which suggest that too much sleep may be associated with the risk of dementia ; however, some of these other studies did not consider other dementia risk factors, such as depression that may lead people to sleep more . It also contradicts another study that reported transitioning from normal sleep duration (6-9 hours per night) to longer sleep duration (>9 hours per night) increased dementia risk by 145% over a 10-year follow-up period .
By their nature, you cannot ascertain cause and effect in epidemiology studies. However, the preponderance of evidence suggests that getting fewer than six hours of sleep per night may be associated with an increased risk of dementia. The strength of this study is that they followed patients for 25 years, made repeated measures of sleep duration, and enrolled individuals in midlife, rather than elderly individuals who may already have developed dementia pathology. The best evidence to date suggests that one should get at least seven hours of sleep per night, and it is one of our “7 Steps to Brain Health”.
Nick McKeehan is a member of the ADDF's Aging and Alzheimer's Prevention program. He evaluates the scientific evidence for and against therapies to promote brain health and/or prevent Alzheimer's disease at our website CognitiveVitality.org and contributes regularly to the site's blog.
Mr. McKeehan previously served as Chief Intern at Mid Atlantic Bio Angels (MABA) and was a research technician at Albert Einstein College of Medicine investigating repair capabilities of the brain. Mr. McKeehan received a bachelor of science degree in biology from Purdue University, where he was awarded a Howard Hughes Scholarship. He also writes about the biotechnology industry for 1st Pitch Life Science.
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