Sleeping for more than 8 hours each night was linked to a 46% increased risk of stroke.
The research team, including Yue Leng of the Department of Public Health and Primary Care at the University of Cambridge in the UK, publishes its findings in the journal Neurology.
According to the National Sleep Foundation, adults aged 18-64 should get 7-9 hours of sleep each night.
But this latest study, involving more than 9,000 people with an average age of 62, found that getting more than 8 hours sleep each night was associated with a 46% increased risk of stroke.
Stroke occurs when the blood supply to the brain is restricted, either through a blood clot or a burst blood vessel. More than 795,000 people in the US experience stroke each year and it is the leading cause of disability in the country.
Based on their findings, Leng and colleagues say it is unclear as to whether sleeping more than 8 hours a night - referred to as "long sleep" - is a cause or result of stroke.
"It is more likely that extended sleep duration could serve as an early warning sign of increased stroke risk, particularly in older people," Leng told Medical News Today. "In any case, the finding should not be interpreted as 'restrict sleep duration so as to cut stroke risk.'"
Could an increase in sleep time be an early warning sign of stroke?
To reach their findings, the team recruited 9,692 people with an average age of 62 who had never experienced a stroke.
Over an average follow-up period of 9.5 years, the participants completed two questionnaires 4 years apart that gathered information on how many hours they slept each night. During follow-up, 346 participants had a stroke.
Compared with participants who slept for 6-8 hours each night - considered an "average" amount of sleep - the team found those who slept for more than 8 hours were 46% more likely to have a stroke.
What is more, the researchers found that people who increased their amount of sleep from 6-8 hours each night to more than 8 hours during follow-up were four times more likely to have a stroke, compared with those who consistently slept for 6-8 hours a night.
An additional analysis of previous research that assesses the link between sleep duration and stroke, conducted by Leng and colleagues, also found that sleeping more than 8 hours a night was associated with increased stroke risk.
In an editorial linked to the study, Dr. Alberto Ramos, of the University of Miami Miller School of Medicine, FL, says:
"Since people whose sleep patterns changed from short to long were nearly four times as likely to have a stroke, it's possible that this could serve as an early warning sign, suggesting the need for additional tests or for people to take steps known to reduce stroke risk, such as lowering blood pressure and cholesterol."
MNT asked Leng what she believed the drivers were behind this association. She told us that the increased sleep duration among participants might have indicated excessive sleepiness and fatigue as a result of declining health.
"Moreover," she added, "the big increase in sleep needs might also reflect alterations in blood flow in the brain and thus serve as an early sign of future stroke risk. However, the exact biological mechanism remains unclear and requires confirmation from physiological studies."
Earlier this month, Medical News Today reported on a study by researchers at Columbia University's Mailman School of Public Health, which found that the average number of hours slept per night among teenagers in the US has declined over the past 2 decades.