U.S. Navy sleep studies have discovered another thing to look for when selecting sailors for different kinds of duty. The study found that short sleepers (those who typically slept for six hours or less a night), were more alert when they were awakened, but were much less alert after 36 hours without sleep. Long sleepers (those who require nine or more hours a night) took longer to become completely alert when they were awakened, but were much more alert after 36 hours or more of being awake. So if you have to wake up people for an emergency that requires concentration, pick a short sleeper. If you need people who might have to stay awake for long periods, and still be able to function, pick a long sleeper.