49 Sleep
Sleep is part of a person's daily activity cycle. There are several different stages of sleep, and they too occur in cycles. If you are an average sleeper, your sleep cycle is as follows. When you first drift off into slumber, your eyes will roll about a bit, your temperature will drop slightly, your muscles will relax, and your breathing well slow and become quite regular. Your brain waves slow down a bit too, with the alpha rhythm of rather fast waves predominating for the first few minutes. This is called stage 1 sleep. For the next half hour or so, as you relax more and more, you will drift down through stage 2 and stage 3 sleep. The lower your stage of sleep, the slower your brain waves will be. Then about 40 to 60 minutes after you lose consciousness you will have reached the deepest sleep of all. Your brain waves will show the large slow waves that are known as the delta rhythm. This is stage 4 sleep.You do not remain at this deep fourth stage all night long, but instead about 80 minutes after you fall into slumber, your brain activity level will increase again slightly. The delta rhythm will disappear, to be replaced by the activity pattern of brain waves. Your eyes will begin to dart around under your closed eyelids as if you were looking at something occurring in front of you. This period of rapid eye movement lasts for some 8 to 15 minutes and is called REM sleep. It is during REM sleep period, your body will soon relax again, your breathing will grow slow and regular once more, and you will slip gently back from stage 1 to stage 4 sleep - only to rise once again to the surface of near consciousness some 80 minutes later.
睡眠
睡眠是人每天日常活动循环的一部分。 人的睡眠分几个阶段，而这些阶段也是循环发 生的。 如果你是一个正常的睡眠者，你的睡眠循环会这样进行。 在你开始昏昏入睡时，你 的眼睛会滚动几下，体温略有下降，肌肉放松，呼吸变得缓慢而有节奏。 除了开始几分钟 比较快的α节奏外，脑电波也稍有减缓。 这被称为第一阶段睡眠。 在随后约半小时内，你 进一步放松，进入第二和第三阶段睡眠。 睡眠越深入，脑电波就越缓慢。 大约在开始睡眠 后的 40 到 60 分钟，你将进入沉睡状态。 这时的脑电波表现为巨大的缓波，被称为δ节奏。 这就是第四阶段睡眠。 但你并不是整夜都保持这种沉睡状态。 入睡后约 80 分钟左右，你 的大脑运动水平会再度略有提高。 δ节奏消失，并被脑电波的运动图形取代。 你的眼睛会 在闭着的眼睑下迅速转动，就好象你在看着眼前发生的什么事情。 这种迅速的眼球运动持 续约 8～15 分钟，这一阶段睡眠被称之为快速眼动(REM)睡眠。 在 REM 睡眠阶段，你的肢 体会很快再度放松，呼吸也再次放慢并变得有节奏，你会轻松地从第一阶段滑入第四阶段睡 眠-直到大约 80 分钟后重新接近清醒状态。
50 Cells and Temperature
Cells cannot remain alive outside certain limits of temperature, and much narrower limits mark the boundaries of effective functioning. Enzyme systems of mammals and birds are most efficient only within a narrow range around 37 ℃; a departure of a few degrees from this value seriously impairs their functioning. Even though cells can survive wider fluctuations, the integrated actions of bodily systems are impaired. Other animals have a wider tolerance for changes of bodily temperature. For centuries it has been recognized that mammals and birds differ from other animals in the way they regulate body temperature. Ways of characterizing the difference have become more accurate and meaningful over time, but popular terminology still reflects the old division into "warm blooded" and "cold blooded" species; warm-blooded included mammals and birds whereas all other creatures were considered cold-blooded. As more species were studied, it became evident that this classification was inadequate. A fence lizard or a desert iguana -- each cold-blooded -- usually has a body temperature only a degree or two below that of h
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