2015考研英语作文素材精选 (十四)

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考研英语作文是一个考察综合运用语言的部分，需要同学们运用逻辑思维下笔成文，因此，考前看一些意义深远、质量好的文章很有必要。以下是2015考研英语作文备考素材精选，请作参考。
   
     2015考研英语作文素材精选 (十四)
    49 Sleep
    Sleet 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 fist drift off into slumber,
your eyes will roll about a bit, you temperature will drop slightly, your
muscles will relax, and your breathing well slow and become quite regular. Your
brain waves slow and become quite regular. Your brain waves slow down a bit too,
with the alpha rhythm of rather fast waves 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. slower your brain waves will be. Then
about 40to 69 minutes after you lose consciousness you will have reached the
deepest sleep of all. Your brain 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 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.
    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 37C;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 humans and so is not cold. Therefore the next distinction
was made between animals that maintain a constant body temperature, called
home0therms, and those whose body temperature varies with their environments,
called poikilotherms. But this classification also proved inadequate, because
among mammals there are many that vary their body temperatures during
hibernation. Furthermore, many invertebrates that live in the depths of the
ocean never experience change in the depths of the ocean never experience change
in the chill of the deep water, and their body temperatures remain constant.