Turning the clocks forward an hour for daylight saving time is followed by a spike in heart attacks on the Monday afterward, according to a US study.
But when the clocks fall back and people gain an hour of sleep, there is a drop in heart attacks on Tuesday, said the research presented on Saturday at the American College of Cardiology conference.
The findings showed no change in the total number of heart attacks during the entire week following any clock change, indicating that the spike seen on Monday is levelled out in the days that follow.
But knowing that a surge in patients can be expected in the emergency room could help doctors better prepare, said lead author Amneet Sandhu, cardiology fellow at the University of Colorado in Denver.
"It may be that we as people are very sensitive to the loss of even one hour's sleep," Sandhu told reporters.
"It may mean that people who are already vulnerable to heart disease may be at greater risk right after sudden time changes."
The study was based on a database of hospitals in Michigan.
There was a 25 per cent jump in the number of heart attacks occurring the Monday after the spring time change - or a total of eight additional heart attacks - and a 21 per cent drop the Tuesday after the return to standard time.
Monday is traditionally the day when most heart attacks occur, previous research has found.
But by looking at hospital data over four consecutive years, researchers saw a consistent 34 per cent increase in heart attacks from one week to the next at the spring time change.
There were an average of 93 heart attacks the Monday before compared to 125 the week after the start of daylight saving time across those four years.
Daylight saving time - implemented to save energy during World War I - is controversial and some believe it is not needed anymore.
Sandhu said future research should compare the Michigan findings to heart attack trends in Hawaii and Arizona, which do not have daylight saving time.
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