单项选择题
| Sleep is something we generally
associate with living creatures. Of course, it is (36) that a
lot of animals sleep, but zoologists are certain that (37)
forms of animal life, (38) worms and snails, never
really sleep. On the other (39) , animals such as bears sleep
for 4 (40) 5 months every year. The (41) of sleep a human being needs (42) on age, the individual and possible (43) . For example, doctors think that pre school children need between 10 and 12 hours a night, school children between 9 and 11 hours, and (44) between 7 and 8 hours. There are exceptional (45) of old people (46) only sleep between 2 and 3 hours a day and (47) to be active and healthy. The sleep (48) of different (49) also appear to be (50) . Japanese people for example, sleep (51) hours than Europeans. (52) is not known for (53) whether mental activity (54) when a person is asleep. However, it is certainly true that some people can wake (55) at a specific, pre-determined time. |