It is fairly（相当） clear that the sleeping period must have some function（运行）, and because there is so much of it, the function would seem to be important. Speculations（思索） about its nature have been going on for literally（照字面地） thousands of years, and one odd（奇数的） finding（发现） that makes the problem puzzling is that it looks very much as if sleeping is not simply（简单地） a matter of giving the body a rest. Rest, in terms of muscle（肌肉） relaxation（松弛） and so on, can be achieved by a brief（简短的） period lying（躺）, or even sitting down. The body's tissues（薄绢） are self（自己）-repairing and self（自己）-restoring to a degree, and function best when more or less continuously active. In fact, a basic（基本的） amount of movement occurs（发生） during sleep, which is specifically（明确地） concerned（有关的） with preventing muscle（肌肉） inactivity.