For many people, getting out of a warm bed in winter is hard. But a new study linking sleeping hours to health risks could sound an alarm for late sleepers.
The study, published in the European Heart Journal, shows people who sleep more than eight hours a day might also have an increased risk of heart and blood vessel disease and even death.
Researchers from China's Beijing Fuwai Hospital and Canada's McMaster University worked on the study, which involved 116,632 people aged 35 to 70 in 21 countries since 2003. Following up after almost eight years, researchers recorded 4,381 deaths and 4,365 cases of cardiovascular disease.
They had taken account of factors that could affect outcomes, such as age, sex, smoking, alcohol consumption, family history of cardiovascular disease, diabetes, and blood pressure.
They found people who slept six to eight hours a day had lowest death and disease rate. Both less and more sleep were associated with an increased risk, but a long sleep was linked to a higher risk than a short sleep.
Eight to nine hours of sleep a day increased the risk by 5 percent; nine to 10 hours by 17 percent; and over 10 hours by 41 percent. However, fewer than six hours, increased the risk by 9 percent.
Researchers also found that daytime napping (about 30 to 60 minutes) could increase the risk except in people sleeping less than six hours at night.
The study has limitations, including the assumption that the duration of sleep did not change during the follow-up period, and they did not collect information on sleep disorders such as insomnia and apnoea, which also had an impact on sleep and might affect health.
But the result is similar to a study by British researcher last month, which showed that women who slept longer than eight hours a night had a 20-percent increased risk of breast cancer for each additional hour slept.
The duration of sleep may help doctors to identify people at high risk of heart and blood vessel problems or death, said Wang Chuangshi, lead author of the study.