Long hours were defined in the French study as more than 10 hours on at least 50 days per year. People who did long hours for more than a decade were at the greatest risk of stroke, they suggest.
The researchers, from Angers University and the French National Institute of Health and Medical Research, looked at data on age, smoking and working hours from a population study of more than 143,000 adults. Just under a third worked long hours, with 10% working long hours for 10 years or more. Overall, 1,224 had had a stroke.
The researchers say people working long hours had a 29% greater risk of stroke, and those doing so for 10 years or more had a 45% greater risk.
Dr Alexis Descatha, who led the research added: "The association between 10 years of long work hours and stroke seemed stronger for people under the age of 50. This was unexpected. Further research is needed to explore this finding."
Other research has found people who run their own businesses, CEOs and managers seem less affected by long hours - as opposed to those working irregular shifts and nights, or who have job-related stress.