A new study suggests that sleeping for more than eight hours a day increases the risk of stroke.

Researchers at the University of Cambridge found that people who sleep longer than eight hours a day were at a 46 percent greater risk of stroke than average. This risk doubles for older people who persistently sleep longer than average.

For the study, researchers followed nearly 10,000 people aged 42 to 81 years of age from the European Prospective Investigation into Cancer (EPIC)-Norfolk cohort for more than nine 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 10 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.

Participants who reported persistently long sleep -- in other words, they reported sleeping more than 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.

Researchers said it is unclear yet why the link between sleep and stroke risk should exist. Lack of sleep has been linked with factors such as disrupted metabolism and raised levels of the 'stress hormone' cortisol, all of which may lead to higher blood pressure and increased stroke risk.

The findings are detailed in the journal Neurology.