Setting clocks ahead an hour in the spring for daylight saving time is followed by an increase in heart attacks on the Monday afterward, according to a new study led by an Indian-origin scientist.
Widely implemented during World War I, daylight saving time was primarily adopted to save energy and help people enjoy more of the morning sun.
The study comes amid ongoing debate whether daylight saving time is actually needed anymore.
US researchers found a 25 per cent jump in the number of heart attacks occurring the Monday after people "spring forward" compared to other Mondays during the year - a trend that remained even after accounting for seasonal variations in these events.
The study showed the opposite effect is also true. Researchers found a 21 per cent drop in the number of heart attacks on the Tuesday after returning to standard time in the fall when we gain an hour back.
"What's interesting is that the total number of heart attacks didn't change the week after daylight saving time," said Amneet Sandhu, cardiology fellow, University of Colorado in Denver, and lead investigator of the study.
"But these events were much more frequent the Monday after the spring time change and then tapered off over the other days of the week.
"It may mean that people who are already vulnerable to heart disease may be at greater risk right after sudden time changes," Sandhu said.
Previous research has shown that heart attacks occur most often on Monday mornings. Sandhu explained that in looking at other "normal" Mondays, there is some variation in events, but it is not significant.
He and his team compared admissions from a database of non-federal Michigan hospitals the Monday before the start of daylight saving time and the Monday immediately after for four consecutive years.
They found a consistent 34 per cent increase in heart attacks from one week to the next (93 heart attacks the Monday before compared to 125 the week after the start of daylight saving time for the duration of the study).
Although researchers cannot say what might be driving the shift in heart attack timing after the start of daylight saving time, they have a theory.
"Perhaps the reason we see more heart attacks on Monday mornings is a combination of factors, including the stress of starting a new work week and inherent changes in our sleep-wake cycle," Sandhu said.
"With daylight saving time, all of this is compounded by one less hour of sleep. Whatever the reason, he said, the findings may indicate a need to better staff hospitals the Monday after setting our clocks forward," Sandhu said.
The study was presented at a meeting of the American College of Cardiology.