People who are deprived ___(1)___ sleep lose energy and become quick-tempered. After two days ___(2)___ sleep, a person finds that lengthy concentration becomes difficult. He can force ___(3)___ to perform well for short periods, but he is easily ___(4)___. He ___(5)___ many mistakes, especially at routine tasks, and his attention slips ___(6)___ times. Every "sleepless" person experiences periods in which he ___(7)___ off for a few seconds or more. He ___(8)___ completely asleep unless he is kept active continuously. Persons who go without sleep for more than three days have great difficulty thinking, seeing, and hearing clearly. They have periods of hallucinations, ___(9)___ they see things that do not really exist. They also confuse day dreams with real life and ___(10)___ track of their thoughts in the middle of a sentence.