Setting the clock ahead for daylight savings time may set the scene for a small increase in heart attacks the next day, according to a U.S. study - which suggests that sleep deprivation may be to blame.
Researchers at two hospitals in the U.S. state of Michigan, whose findings appeared in the American Journal of Cardiology, reviewed six years of records and found that they treated an average of 23 heart attacks on the Sunday when the United States switched to daylight savings time. That compared to 13 on a typical Sunday.
"Nowadays, people are looking for how they can reduce their risk of heart disease and other ailments," said Monica Jiddou, the study's lead author and a cardiologist at William Beaumont Hospital in Royal Oak.
"Sleep is something we can potentially control. There are plenty of studies that show sleep can affect a person's health."
A 2008 Swedish report, for instance, found that the chance of a heart attack increased in the first three weekdays after the switch to daylight savings time, and decreased the Monday after the clocks returned to standard time in the autumn.
Jiddou told Reuters Health that her team wanted to see if their respective hospitals experienced the same increase and decrease in heart attacks seen in the Swedish study.
For the new study, she and her colleagues reviewed records for the 328 patients who were diagnosed with a heart attack during the week after a time change between 2006 and 2012, and for the 607 heart attack patients who were treated two weeks before and after the time shifts.
They found that except for the small increase on the Sunday that daylight savings time kicked in, there were no significant differences in heart attack rates in the first week after the spring clock change or in the fall, when people set clocks back.
The authors note, however, that the small trends they observed suggest shifts to and from daylight savings time may be linked with small increases in heart attacks in the spring, and small decreases in the fall.
They speculate that sleep deprivation resulting from the time changes could raise levels of stress hormones and inflammatory chemicals just enough to trigger a heart attack, especially in those already at high risk.
Though the slight increase in heart attacks in the days following time shifts were so small they could have been due to chance, Jiddou told Reuters Health that she believes the problem was the size of the study population.
"The numbers weren't necessarily striking, but the trends make you stop and think," she said.
But Steven Nissen, a cardiologist who is chair of the Robert and Suzanne Tomsich Department of Cardiovascular Medicine at the Cleveland Clinic, said that people should be carefully interpreting the findings.
"We haven't generally thought that missing an hour of sleep causes heart attacks. This may or may not hold up," Nissen said.
He added that while the study looks at a good question and he applauds the researchers' efforts, but stressed the limitations of the results and noted that the size of the effect is not huge.