Pauline Anderson

March 09, 2016

Turning clocks ahead an hour in the spring or turning them back again in the fall is associated with an increased incidence of ischemic stroke, at least temporarily, a new study shows.

The increased stroke risk just after a transition to and from daylight saving time is likely related to interrupted sleep patterns, said study author Jori Ruuskanen, MD, PhD, University of Turku, Finland.

He noted that previous studies have shown that shift workers, and others whose sleep may be fragmented, also have increased risk for stroke.

Dr Jori Ruuskanen

"We don't know what the mechanism behind this is, but what is common is the disturbed sleep," he told Medscape Medical News.

Their findings were released February 29 and will be presented at the upcoming American Academy of Neurology (AAN) 2016 Annual Meeting in April.

Circadian rhythm disruption has been associated with increased risk for ischemic stroke, the authors note. It has been shown previously that daylight saving time transitions disrupt circadian rhythms and can shift the pattern of diurnal variation in stroke onset, they add, but its effects on the overall incidence of ischemic stroke are unclear.

For this analysis, the researchers studied the effects of transitioning to and from daylight saving time on stroke hospitalizations across Finland from 2004 to 2013. They compared the incidence of ischemic stroke (IS) among 3033 patients hospitalized during the week after the daylight saving transition with the stroke rate among a control group of 11,801 patients hospitalized 2 weeks before and 2 weeks after that index transition week.

They found that the incidence of ischemic stroke increased during the first 2 days after transition (incidence ratio, 1.08; 95% confidence interval [CI], 1.01 - 1.15; P = .020).

Although the rate fluctuated somewhat later during the week of transition, the effect was "diluted" after 2 days upon looking at the whole week, presumably as participants started to adapt to the change, said Dr Ruuskanen.

There was no difference between men and women in the overall risk for stroke during the week after the transition. There was also no difference between those living in the northern part of the country, which is more rural, and in the more urban southern areas, said Dr Ruuskanen.

Patients With Cancer

There was an even higher increased risk for stroke in the first 2 days after a transition among patients with malignancies (risk ratio [RR], 1.25; 95% CI, 1.00 - 1.56; P = .047).

"Cancer in itself is a risk factor for stroke," said Dr Ruuskanen. "People with cancer, as well as elderly people, may be more prone to sleep difficulties, but there may also be other factors, such as changes in immunology, which interplay with sleep disturbance to increase the risk further."

The study also found the increased risk was more pronounced among those older than 65 years (RR, 1.20; 95% CI, 1.04 - 1.38; P = .020).

No comorbidities other than cancer appeared to be associated with increased stroke risk during the daylight saving time transition, said Dr. Ruuskanen.

Researchers also looked at the effect of daylight saving time changes on in-hospital mortality but found no effect.

Increased risk did not differ between turning the clocks back (when people gain an hour and so presumably get more sleep) and turning them forward (when an hour is lost).

"We didn't see any difference in relative risk of IS after the daylight saving time transitions between spring and autumn periods, which is a bit surprising, but I think the body will react to both changes," said Dr Ruuskanen.

An interesting question is whether people in areas that don't have daylight saving time (the province of Saskatchewan in Canada, for example) have fewer strokes. "That would certainly be interesting to look at," said Dr Ruuskanen. However, he doesn't think that would have a big effect on the stroke incidence in the grand scheme of things.

"This is only a transition; it's a relatively small and temporary change," he said.

Occurring Earlier

It might be that daylight saving time transition makes strokes that are inevitable occur earlier, said Dr Ruuskanen. "What it probably means is that some strokes appear a bit earlier than they otherwise would have. We can't say for sure that this will cause any extra strokes."

Dr Ruuskanen noted that his colleagues have completed a study showing that daylight saving time transitions changed the occurrence pattern of myocardial infarction, although not the overall incidence of myocardial infarction (Ann Med. Published online December 17, 2015).

Commenting on the research for Medscape Medical News, Ralph Sacco, MD, Department of Neurology, University of Miami Miller School of Medicine, Florida, said he found the study "intriguing."

However, the authors can only speculate on the reasons for the association between a greater stroke risk within the 2 days of moving the clocks forward or backward because the study data can't really address any causal associations, said Dr Sacco.

"There has been an increasing appreciation that sleep patterns, including too much or too little sleep, may alter stroke risk. This study raises more questions and requires more confirmatory studies before we make any conclusions about the stroke risks associated with moving our clocks."

The study was supported by the Finnish Cardiac Society.

American Academy of Neurology (AAN) 2016 Annual Meeting. Abstract 2952.

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