In fact, laboratory tests show that sleepy people, when kept busy, are almost as effective as wellrested people. You couldn't even tell a sleepdeprived air traffic controller if you looked at him for 30 seconds in a crisis situation. The problem with sleepy people is what happens when they're not busy, when there's no crisis, when the job is routine. They may miss, or may misinterpret, the early signs of a problem, allowing a minor mistake or malfunction to balloon into a catastrophe. Or, they may simply fall asleep and not realize it later.
People have what are referred to as microsleeps.
They sleep for seconds or perhaps minutes. Cousins told the safety board he was wellrested as he worked on the Exxon Valdez in the hours leading up to the crash, but he was unable to explain what happened during a critical five minutes just before the accident. Cousins was ordered by Hazelwood to start turning away from Bligh Reef and back toward the standard tanker lanes when the ship passed Busby Island, a little more than two miles north of the reef. At the safety board hearings, Cousins was asked to explain the gap. Then he was asked if he could simply have lost track of time for a few minutes.