Beware Possible Clickbait from New Sleep Study
A systematic review and meta-analysis recently published to Scientific Reports shows that sleeping longer than seven hours a night is associated with an increased risk of death. Scientists from Qingdao University in China pooled 35 studies examining the link between sleep duration and death, featuring over 1.5 million individuals in total from all across the globe. Subjects who slept longer had a greater risk of mortality. I can see the clickbait now.
"Too much sleep could kill you, claims study."
"Turns out sleeping in could kill you."
"Science Says Getting More Than 8 Hours Of Sleep May Cause Early Death."
Unfortunately, these predictions weren't conjured from thin air. These are headlines and opening paragraphs from media reports covering other sleep studies that returned similar results. All, in my opinion, did not give enough weight to the ginormous elephant in the room: reverse causality.
A glance at this basic, yet macabre graph can easily lead one to believe that the more sleep you get over seven hours, the more likely you are to die -- 7 percent more likely at eight hours, 21 percent more likely at nine hours, and 37 percent more likely at ten hours. Cue hype. You'd be better off getting four hours of sleep per night than nine hours. More sleep is killing us, and scientists can't explain why!
But far more likely is a simpler explanation: people who are already in poor health -- and thus more likely to die -- sleep longer than healthy individuals. Think about it. What are the two most common side effects of most illnesses? Tiredness and fatigue!
In a 2013 study where researchers carefully controlled for baseline health, there were no significant differences in mortality for varying sleep durations.
To their credit, the authors of the analysis pointed out the glaring limitation.
"One major concern with the observed associations is that short and long sleep duration might be just markers of poor health status rather than independent predictors."
Their worries echo the cautionary wisdom of Francesco Cappuccio, a Professor of Cardiovascular Medicine & Epidemiology at the University of Warwick, and the architect of a previous meta-analysis on the subject.
"Whilst short sleep may represent a cause of ill-health, long sleep is believed to represent more an indicator of ill-health."
Closer examination of the data in the present meta-analysis reveals a number of quirky details. For example, the studies conducted in Asia showed much greater risks from increased sleep duration than studies conducted in Europe or the United States. At ten hours of sleep, Asian subjects had a 70 percent increased risk of dying, compared to just 26 percent for European subjects, and 11 percent for Americans. Why the disparity? Moreover, studies of lower quality, which contributed most of the data, showed even higher mortality risks for subjects who slept longer than seven hours and very low mortality risks for subjects who slept fewer than seven hours. That's fishy.
Of course, it is entirely possible (albeit unlikely) that more sleep -- which many people would kill for -- may be killing us. Wouldn't that be ironic?
Source: Shen, X. et al. Nighttime sleep duration, 24-hour sleep duration and risk of all-cause mortality among adults: a meta-analysis of prospective cohort studies. Sci. Rep. 6, 21480; doi: 10.1038/srep21480 (2016).
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