Try to rest easy after reading this. The UK's Daily Telegraph has reported on its website that regularly sleeping for more than eight hours, or less than six, could lead you to an early grave. According to a decade-long study by researchers at Warwick University, in Coventry, England, the optimum number of hours an adult should sleep each night is between seven and eight .More than a million people were asked about their sleeping habits.According to the Daily Telegraph, sleeping too little or too much can lead to diabetes, heart disease and other maladies.Franco Cappuccio, a professor of cardiovascular medicine and epidemiology, said those who overslept had a 30% higher risk of death than medium-sleepers, the research seemed to show.Those who under-slept had a 12% higher mortality risk.But Cappuccio said that the figures should be taken with "a pinch of salt".He noted that those surveyed might have confused time in bed with time asleep, or had failed to take naps into account...