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 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.
But the risk was similar for those who increased to at least eight hours.
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.
The researchers took into account other possible factors such age, sex, marital status, employment grade, smoking status and physical activity.
Once they had adjusted for those factors they were able to isolate the effect that changes in sleep patterns over five years had on mortality rates 11-17 years later.
Those who cut their sleeping from seven to five hours a night had twice the risk of a fatal cardiovascular problem of those who stuck to the recommended seven hours a night - and a 1.7 increased risk of death from all causes.
--Disturbed sleep common
Researcher Professor Francesco Cappuccio said: ""Fewer hours sleep and greater levels of sleep disturbance have become widespread in industrialized societies.
""This change, largely the result of sleep curtailment to create more time for leisure and shift-work, has meant that reports of fatigue, tiredness and excessive daytime sleepiness are more common than a few decades ago.
""Sleep represents the daily process of physiological restitution and recovery, and lack of sleep has far-reaching effects.""
Curiously, the researchers also found that those individuals who increased their sleep to eight hours or more a night were more than twice as likely to die as those who had not changed their habit.
Professor Cappuccio said lack of sleep had been linked to an increased risk of weight gain, high blood pressure and type 2 diabetes.
However, he said the link between too much sleep and poor health was less clear, although he suggested that staying in bed for prolonged periods could be a sign of depression, or, in some cases, cancer-related fatigue.
""Our findings indicate that consistently sleeping around seven hours per night is optimal for health and a sustained reduction may predispose to ill-health.""
Dr. Neil Stanley, a sleep expert from Norwich and Norfolk University Hospital, said while public health messages focused on diet and exercise, people were given very little information about the need to get proper amounts of sleep.
""This study is yet more evidence of the importance of getting sleep, and the right amount of sleep for you,"" he said.
""Sleep need is like height or shoe size: we all have an individual one, and we sleep less or more then there are consequences to pay.""