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This article was originally published on Feb. 29, 2016. 

Gut Check is a periodic look at health claims made by studies, newsmakers, or conventional wisdom. We ask: Should you believe this?

The Claim: The change to daylight saving time can kill you, or at least give some people heart attacks and strokes.

The Background: Scientists have documented that messing with the body’s circadian clock — its brain-based timekeeper — disrupts physiology. That’s been shown most definitively with shift work, in which being awake when the sun and body say “night” and asleep when both say “day” increases the risk of hypertension and cardiovascular disease. For instance, a study last month by researchers at Brigham and Women’s Hospital found that day-night reversal increased blood pressure and raised levels of inflammatory molecules.


But just because a 12-hour time shift can cause physiological effects that raise the risk of heart attacks and stroke — as those changes do — doesn’t mean a measly one-hour change does. Still, there have been some hints of that.

A 2013 study of nearly 1,000 patients at two Michigan hospitals compared admissions for heart attacks during the seven days after the move to daylight saving to the same days two weeks prior. In the study, which looked at data between 2006 and 2012, researchers found 17 percent more heart attacks after “springing ahead,” with a 71 percent spike on the first day, Sunday. In fact, that one day accounted for almost all of the overall increase.


It may seem surprising that people would be affected on Sunday, which for most is not a workday. But even partial sleep deprivation can increase levels of adrenaline and a related molecule that increases the risk of heart attack, said lead author Dr. Monica Jiddou-Patros, a cardiologist at William Beaumont Hospital. Emotional stress — contemplating how badly you’ll feel getting up for work on Monday when it feels like 6 a.m. but the infernal clock says 7 — can make platelets aggregate and the blood more likely to coagulate, both of which can make arterial plaques rupture and trigger a heart attack.

Other research has found a daylight-saving effect, too. In two Swedish studies, one in 2008 and one in 2012, the incidence of heart attacks was about 5 percent higher during the first week after clocks changed, mostly in the first three days afterward.

A 2014 study confirmed these findings in a larger and more heterogeneous population. Combing through a multihospital database of 42,060 heart attacks requiring angioplasty from 2010 to 2013, researchers led by Dr. Hitinder Gurm of the University of Michigan found that the total number didn’t change during the first week of DST. But the heart attacks seemed to shift forward. That is, back-to-work Monday after the time change brought 24 percent more heart attacks than usual, about 171 versus 110. But the rest of the week saw fewer than average. (Mondays generally see more heart attacks than other days — again, likely due to stress — but the post-DST Monday was even higher.) “It suggests that the switch to daylight saving makes vulnerable people have heart attacks sooner,” possibly due to circadian rhythms being disrupted (even if you go to bed on “new time”) and the stress of needing to adjust to rising earlier, Gurm said. “I don’t know about other people, but I feel terrible on that Monday.”

First Take: A study presented last month at the annual meeting of the American Academy of Neurology supports the idea that switching to daylight saving time pulls cardiovascular events forward, basically causing people who are on track for one to have it sooner rather than later.

Analyzing a decade worth of strokes in Finland, scientists found a brief spike in the incidence of ischemic stroke (the most common kind, caused by a clot blocking blood flow in the brain) after the clocks are turned ahead compared to the week before. The rate was 8 percent higher during the first two days after setting the clocks ahead, Dr. Jori Ruuskanen of Finland’s University of Turku and colleagues reported. But in people over 65, the incidence of stroke on those Sundays and Mondays was 20 percent higher. It returned to baseline in the following days, suggesting that people adjust to the time change quickly and that moving the clocks ahead caused few if any strokes that would not have happened anyway.

Second Take: The association between moving to daylight saving time and severe cardiovascular events is the kind of question that is vulnerable to publication bias: researchers might be more likely to write up (and journals are more likely to publish) a study that finds a link than one that doesn’t. Another concern is that the published studies so far, although large (thousands of strokes or heart attacks), have been on populations that might not be representative — Swedes, Michiganders, Finns.

Ideally, scientists interested in the question would also study, say, residents of Hawaii and the parts of Arizona which are DST refusniks; that would help rule out the possibility that something else about that period in March increases the incidence of heart attacks and strokes. It would also help to compare the effect of DST on working people to those who can sleep in, and on those who go to sleep at “old” time versus “new.”

The Takeaway: There is suggestive but not ironclad evidence that people who are at risk for heart attacks and strokes have a greater chance of having one in the days right after the clocks move ahead, but the risk to any individual is small, and it’s unlikely that DST is causing cardiovascular events that would not have happened anyway.