Science1 publisher2 min readPublished
York pooled analysis of 69 studies links sleep outside 7 to 8 hours to higher dementia risk
York University researchers pooling 69 studies linked sleeping under 7 hours to 18% higher dementia risk and sleeping over 8 hours to a 28% higher risk. The authors stress that the pooled cohorts show associations and do not prove cause and effect.
The Scientist · Science desk
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What happened
- The same analysis linked less than 150 minutes of physical activity a week and more than 8 hours a day of sitting to a significant rise in dementia risk.
- The pooled studies covered almost 4.5 million people, some as young as 35, all followed from a dementia-free start to see who went on to develop the disease.
- Only 3 of the 69 studies looked specifically at how much time people spent sitting.
- The reviewed studies differed significantly in how they collected and compiled their data.
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Why it matters
- constraint The 28% long-sleep figure cannot be read as a measured harm of oversleeping while excess sleep may itself be an early sign that Alzheimer's has begun.
- constraint With sitting data from about 4% of the pooled studies, the 8-hour sitting cutoff is the threshold most likely to shift as more studies measure it.
- contradiction ScienceAlert's headline promises an 'optimal amount' of sleep; the authors wrote only that the three habits 'may be associated' with reduced dementia risk.
Every cohort in the pool started with dementia-free participants [6]. That removes people who already had a diagnosis. It cannot remove people whose disease had begun without one. For sleep, that gap matters. ScienceAlert's report points to research suggesting that sleeping too much can be a sign that Alzheimer's has already started, not only a driver of it, and notes that telling triggers from consequences in data like these is tricky [8].
Risk was lowest at 7 to 8 hours a night [3]. Long sleep carried the larger association, 28% against 18% for short sleep, a gap of 10 percentage points [4][16]. Suppose some of the long sleepers were already in the early stages of disease when follow-up began. Then part of that 28% is a symptom being counted as a risk factor, and a pooled analysis of observational cohorts has no clean way to measure how large that part is. I'd treat the 18% short-sleep figure as the easier of the two to interpret, on the condition that it is still an association.
The thing these figures don't tell you is how many extra cases they mean. The report gives them as percentage increases in risk without the underlying dementia rate in the cohorts, so an 18% rise cannot be converted into cases per thousand people [4].
The proposed biology was not tested here. The York team, publishing in PLOS One [14], ran no experiments of its own. It points to exercise, movement and sleep helping to keep blood flowing in the brain, clear waste from neurons and protect organs linked to the brain, such as the heart [11]. "Taken together, a healthy movement behavior profile may promote vascular, neurotrophic, and anti-inflammatory effects that collectively support brain health and may delay the onset or progression of dementia," the researchers wrote [15].
The design the authors want next would follow people while their habits change. "Future studies with middle-aged adults and longer-term follow-up, including changes in movement behaviors over time, are needed to better understand the relationship between physical activity, sedentary behavior, and sleep for dementia risk," they wrote [13].
What to watch
- Analyses that drop the first years of follow-up would show how much of the 28% long-sleep link survives once early, undiagnosed disease is set aside.
- New cohorts that measure daily sitting time would show whether the 8-hour sitting link holds beyond the 3 studies now behind it.