The Earths daily clock, measured in a single revolution, is twenty-four hours. The human clock,【B1】______, is actually about twenty-five hours. Thats【B2】______scientists who study sleep have determined from human subjects who live for several weeks in observation chambers with no 【B3】______ of day or night. Sleep researchers have 【B4】______ other surprising discoveries as well. We spend about one-third of our lives asleep, a fact that suggests sleeping,【B5】______eating and breathing, is a fundamental life process. Yet some people almost never sleep, getting by on as 【B6】______ as fifteen minutes a day. And more than seventy years of【B7】______into sleep deprivation, in which people have been kept 【B8】______ for three to ten days, has 【B9】______ only one certain finding: Sleep loss makes a person sleepy and thats about all; it causes no lasting ill【B10】______. Too much sleep, however, may be【B11】______for you. These findings【B12】______some long-held views of sleep, and they【B13】______questions about its fundamental purpose in our lives. In【B14】______, scientists dont know just why sleep is necessary. Some scientists think sleep is more the result of evolutionary habit than【B15】______actual need, Animals sleep for some parts of the day perhaps because it is the【B16】______thing for them to do: it keeps them【B17】______and hidden from predators; its a survival tactic. Before the advent of electricity, humans had to spend at least some of each day in【B18】______and had little reason to question the reason or need for【B19】______. But the development of the electroencephalograph and the resulting discovery in 1937 of dramatic【B20】______in brain activity between sleep and wakefulness opened the way for scientific inquiry in the subject. 【B1】