Does Daylight Saving Time Cause Heart Attacks?

Full transcript

Every spring, the clock change gets blamed for heart attacks.

In many countries, clocks jump forward one hour in spring. Most people lose an hour of sleep that night. A famous talk on sleep, watched tens of millions of times, says this lost hour is dangerous. It says heart attacks rise twenty-four percent the next day. In autumn, when people gain the hour back, it says they fall twenty-one percent.

Those numbers are real, but they come from one study. Researchers in the American state of Michigan counted patients arriving at hospitals, day by day, over about four years. The jump was on a single day, the Monday after clocks moved forward.

But the same study found something the talk did not mention. Over the whole week, the total did not go up. The researchers concluded that the time shift seemed to affect when some patients arrived, not how many.

In twenty twenty-four, a review combined twelve studies from ten countries, to see the bigger picture. In the weeks after the spring shift, it found a much smaller increase, about four percent. For autumn, it found none.

Then, in twenty twenty-five, doctors at Duke University ran one of the largest studies yet. They followed almost one hundred seventy thousand patients across ten years. The doctors compared every day of the clock change week with the week before and the week after. In spring and in autumn, they found no rise at all. They say earlier studies may have been thrown off because they were smaller and covered fewer places.

So if the time shift affects the heart at all, the effect is small, far smaller than the famous number.

Here is our take. Sleep still matters. Regularly short sleep is linked to heart disease and high blood pressure. What counts is the sleep you get every night, not one hour, once a year.

And when you hear a scary number from one study, treat it as a starting point. Before you believe it, check whether bigger studies found the same thing.

One study said twenty-four percent. Twelve studies said about four. A study of almost one hundred seventy thousand patients found none.