People who sleep for more than eight hours a day may be at an increased risk of stroke, a new Cambridge study has found.
This risk doubles for older people who persistently sleep longer than average, researchers said.
The researchers said it is unclear why this association exists and call for further research to explore the link.
Researchers from the Department of Public Health and Primary Care at the University of Cambridge, UK, followed just under 10,000 people aged 42-81 years of age from the European Prospective Investigation into Cancer (EPIC)-Norfolk cohort over 9.5 years.
During 1998-2000 and then again four years later, they asked the cohort how many hours on average they slept in a day and whether they generally slept well.
Almost seven out of ten participants reported sleeping between six and eight hours a day, whilst one in ten reported sleeping for over eight hours a day.
Participants who slept for less than six hours or more than eight hours were more likely to be older, women and less active.
Over the almost ten year period of the study, 346 participants suffered a stroke, either non-fatal or fatal stroke.
After adjusting for various factors including age and sex, the researchers found that people who slept longer than eight hours a day were at a 46 per cent greater risk of stroke than average.
People who slept less than six hours a day were at an 18 per cent increased risk, but the small number of people falling in this category meant the association was not statistically significant, researchers said.
Participants who reported persistently long sleep - in other words, they reported sleeping over eight hours when asked at both points of the study - were at double the risk of stroke compared to those with persistently average sleep duration (between six and eight hours a day).
This risk was even greater for those whose reported sleep increased from short to long over the four years - their risk was close to four times that of people who maintained an average sleep duration.
In addition to studying the EPIC-Norfolk cohort, the researchers carried out a study of combined data from 11 other studies related to identify the association between sleep duration and patterns of stroke risk.
Their final analysis, including 560,000 participants from seven countries, supported the findings from the EPIC-Norfolk cohort study.
"It's apparent both from our own participants and the wealth of international data that there's a link between sleeping longer than average and a greater risk of stroke," said Yue Leng, PhD candidate at the university.
"What is far less clear, however, is the direction of this link, whether longer sleep is a symptom, an early marker or a cause of cardiovascular problems," Leng added.
The study is published in the journal Neurology.
