Researchers say both too much and too little sleep is linked to a doubled risk of fatal cardiovascular disease.
Teams from the University of Warwick and University College London examined sleep patterns and death rates over two decades among 10,308 civil servants.
They found a doubled risk among those who cut their sleeping from seven to five hours a night compared to those who stuck to seven hours a night.
The research, to be presented to the British Sleep Society, was based on data taken in 1985-88 and on follow up information collected in 1992-93. Click here for full report.