Personal Health Navigator

Does switching to daylight saving time put your health at risk?

Written by Paul Taylor

Question: Every spring, when we switch to daylight saving time, the news media are filled with dire warnings about the health risks of losing an hour of sleep. Are these risks for real?

Answer: It’s certainly worthwhile doing a reality check on our commonly held beliefs.  Sometimes, we discover they’re not founded on the best of evidence.

In the case of daylight saving time – which begins March 10  –  it’s often reported that traffic crashes and heart attacks increase in the days immediately after the clocks have been turned forward when people are still adjusting to the time change.

So, let’s examine both of these claims:

More traffic crashes

The most famous study to suggest a link between traffic accidents and daylight saving time was led by Dr. Stanley Coren at the University of British Columbia.

By analyzing data from the Canadian Ministry of Transport, Dr. Coren and his research colleagues concluded that traffic accidents rose by about 8 per cent on the Monday after the clock is turned forward. They also found accidents fell by roughly the same amount on the Monday following the autumn switch back to standard time when people gain an extra hour of sleep.

He attributes the spring spike in crashes to “lapses of attention” resulting from disrupted sleep.

The findings were published in 1996 in a letter to the prestigious New England Journal of Medicine.

“That medical journal has a lot of clout,” say Dr. Donald Redelmeier, a staff physician at Sunnybrook Health Sciences Centre.

Although the study attracted much attention, and continues to be widely quoted, it was based on just two years of data – 1991 and 1992.

Since then, other researchers have investigated the same issue – with varying results.

Dr. Redelmeier, who has a special interest in traffic research, has tallied up 16 separate studies.

“Of those studies, six suggested the springtime shift forward is associated with an increased risk of traffic crashes. Three suggested a deceased risk. And seven found no significant difference one way or the other,” he says.

“Looking at the sum total of evidence – and not just one cherry-picked study – my impression is that, if there is an association, it is modest.”

Increase in heart attacks

The popular notion that seasonal time changes boost the risk of having a heart attack can also be traced to a letter in the New England Journal of Medicine.

That letter, published in 2008, came from researchers at the Karolinska Institute in Stockholm. They reviewed Swedish health data from 1987 to 2006 and found that heart attack cases increased by about 5 per cent in the week after the clocks were adjusted – both in the spring and the autumn.

They speculated that the time changes disturb the  “chronobiology rhythm” – or the body’s internal clock – and this somehow triggers a heart attack.

Other researchers have done follow-up studies that produced conflicting results.

For instance, German researchers found “no significant change” in heart attacks in the general population following seasonal time changes. But they did observe an increased risk during the spring transition among “specific subgroups,” such as men who had previously experienced heart attacks. Their findings were based on 25,000 cases of fatal and non-fatal heart attacks recorded in Germany between 1985 and 2010.

It’s worth noting that it is not always possible to add up all the studies and create a scorecard with a clear result. That’s because the total body of research on some topics can be skewed by what is known as “publication bias,” says Dr. Dennis Ko, a cardiologist at Sunnybrook.

In particular, many studies that fail to show an effect don’t end up in the medical journals. “We tend to publish only the stuff that is interesting and will catch people’s attention. The papers that don’t find an association are less likely to be published,” explains Dr. Ko.

That can lead people to conclude certain things have a greater impact than they actually do.

Dr. Ko thinks the association between daylight saving time and heart attacks is real.  But, he adds, that risk is likely small and probably affects only those with existing heart disease.

Even so, you’re bound to hear the same alarming health warnings whenever we change our clocks.

Yet if you consider all the published research – and keep in mind that some negative (no-effect) studies never see the light of day – the heightened concern may not seem justified.

As Dr. Redelmeier puts it: “One outlier study can sustain a lifetime of belief in an association. It then becomes extremely difficult to dispel the popular misconception.”

About the author


Paul Taylor

Paul Taylor, Sunnybrook’s Patient Navigation Advisor, provides advice and answers questions from patients and their families, relying heavily on medical and health experts. Email your questions to
and follow me on Twitter @epaultaylor