Such mental fatigue can be as threatening as a heart attack. Recent evidence indicates that sleepiness is a leading cause of traffic and industrial accidents. 'Human error causes between 60% and 900 of all workplace accidents, depending on the type of job, ' observes biological psychologist David Dinges of the University of Pennsylvania. 'And inadequate sleep is a major factor in human error, at least as important as drugs, alcohol and equipment failure.' Other research suggests that sleep loss contributes to everything from drug abuse to poor grades in school. A typical adult needs about eight hours of shut-eye a night to function effectively. By that standard, millions of Americans are for a long time sleep deprived, trying to get by on six hours or even less. In many households, cheating on sleep has become an unconscious and harmful habit. 'In its mild form, it's watching Ted Koppel and going to bed late and then getting up early to get to the gym, 'says Cornell's Pollak. In extreme cases people stay up most of the night, seeing how little sleep will keep them going. They try to compensate by short sleeping late on weekends, but that makes up for only part of the shortfall. Over the months and years, some researchers believe, the imbalance builds up and the effects accumulate. 'Most Americans no longer know what it feels like to be fully alert, ' contends Dr. William Dement, director of Stanford University's Sleep Center. They go through the day in a sort of twilight zone the eyes may be wide open, but the brain is partly shut down. Single parent Dianna Bennett, 43, works as a nurse at a correctional facility in Gardner, Mass. To be able to spend time with her three children during the day, she works the night shift, a schedule that usually allows her no more than four hours of sleep, 'My kids tell me I'm always tired,' she says. Amy Schwartzman, 35, a law student at Tulane University, gets up at 9 a.m. and, what with classes, simulated court and work as a research assistant, often does not get home until 10 p.m. That's when she studies or relaxation. Nights of throwing into bed at 3 a.m. make her feel 'as if my brain isn't moving as quickly as quickly as it should.' says Schwartzman, noting that the circles under her eyes keep getting darker. 'My mother told me I look like a raccoon.' Sleep loss contributes to everything except______.