Judging from recent surveys, most experts in sleep behavior. agree that there is virtually an epidemic of sleepiness in the nation. 'I can't think of a single study that hasn't found Americans getting less sleep than they ought to,' says Dr David. Even people who think they are sleeping enough would probably be better off with more rest. The beginning of our sleep-deficit crisis can be traced to the invention of the light bulb a century ago. From diary entries and other personal accounts from the 18th and 19th centuries, sleep scientists have reached the conclusion that the average person used to sleep about 9. 5 hours a night. 'The best sleep habits once were forced on us, when we had nothing to do in the evening down on the farm, and it was dark.' By the 1950s and 1960s, that sleep schedule had been reduced dramatically, to between 7.5 and 8 hours, and most people had to wake to an alarm clock. 'People cheat on their sleep, and they don't even realize they're doing it,' says Dr David. 'They think they're okay because they can get by on 6.5 hours, when they really need 7.5, 8 hours or even more to feel ideally vigorous.' Perhaps the most merciless rubber of sleep, researchers say, is the complexity of the day. When ever pressures from work, family, friends and community mount, many people consider sleep the least expensive item on his programme. 'In our society, you're considered dynamic if you say you only need 5.5 hours' sleep. If you've got to get 8.5 hours, people think you lack drive and ambition.' To determine the consequences of sleep deficit, researchers have put subjects through a set of psychological and performance tests requiring them, for instance, to add columns of numbers or recall a passage read to them only minutes earlier. 'We've found that if you're in sleep deficit, performance suffers,' says Dr David. 'Short-term memory is weakened, as are abilities to make decisions and to concentrate.' People in the 18th and 19th centuries used to sleep about 5 hours a night because they had ______.