Older people who sleep for more than eight hours a day over prolonged periods of time face a 46% greater risk of stroke than average, the researchers found.
The results suggest oversleeping could be an early indicator of stroke in later life, but the authors say further research will be needed to confirm this.
Lack of sleep has been linked to disrupted metabolism and raised levels of stress, but the researchers, from the University of Cambridge, said it was currently unclear how too much sleep could influence stroke risk.
They looked at data from surveys that asked almost 10,000 patients aged 42-81 years about their sleeping habits in 1998-2000 and again four years later, in 2002-2004.
Just one in 10 said they usually slept for more than eight hours a day. Seven out of 10 reported sleeping six to eight hours a day and the rest said they slept less.
The health of the participants was monitored for 9.5 years afterwards, during which time, 346 had a stroke. Those who reported sleeping for longer had a 46% greater risk of stroke, while those who reported sleeping for longer at both points in the study were twice as likely to have a stroke as those who reported persistently average sleep duration.
The authors ran a follow-up analysis study, pulling in international data from 11 other studies. The results supported their findings.
They now hope to investigate the underlying reasons behind the link.
Lead author professor Kay-Tee Khaw said: 'We need to understand the reasons behind the link between sleep and stroke risk.
'With further research, we may find excessive sleep proves an early indicator of increased stroke risk, particularly among older people.'
Fellow researcher Yue Leng said: 'It's apparent from our participants and the wealth of international data that there's a link between sleeping longer than average and a greater risk of stroke.
'What is far less clear, however, is the direction of this link, and whether longer sleep is a symptom, an early marker or a cause of cardiovascular problems.'