University of Warwick researchers analysed 16 studies including sleep data for more than a million people.
Professor Franco Cappuccio found that people who sleep more than eight hours are at risk of dying earlier than those who sleep for six to eight hours.
Professor Cappuccio looked at long-term studies analysing the mortality of people according to how much they slept.
After ten years, 12% of people who slept less than six hours had died, but 30% of those who slept more than eight hours had died.
The increase in risk of death is equivalent to drinking several cups of coffee per day, the researchers warned.
Cappuccio corrected his data to account for people who are depressed, or using sleep medication, and believes that people who sleep more than eight hours per night may have a serious underlying illness which is causing them to sleep longer, thereby slowly killing them.