单项选择题

All mammals require sleep; it is an essential part of life. For giraffes, two hours a
Line day is enough. For bats, that number is closer to twenty. The average adult human
needs between seven and nine hours of sleep every day. Despite these differences,
mammals all have one thing in common: If they do not get enough sleep, they can
(5) suffer serious mental and physical consequences.
The most extensive sleep research has been conducted on people. Through
these studies, scientists have identified two broad categories of sleep, which can
be further divided into five stages with distinct physiological functions. Taken
together, they form a complete sleep cycle. The first four stages are marked by a
(30) lack of rapid eye movement (REM). Therefore, they are referred to as non-REM
sleep. While people are engaged in non-REM sleep, several things are occurring.
Researchers have discovered that during this early sleep, the human body
releases a series of hormones responsible for the proper functioning of certain
body systems. Disrupted sleep can cause these hormones to fall out of balance.
(15) The results can contribute to disorders such as diabetes, depression, infertility,
or even cancer. Lack of sleep, especially in stages 3 and 4, can literally stunt the
growth of children and may increase rates of obesity in adults.
Stage 5 sleep is defined by the occurrence of rapid eye movement, otherwise
known as dreaming. Note that, while REM sleep is the final stage in a sleep cycle,
(20) a complete cycle generally lasts only about 95 minutes. A person will experience
four to six cycles in one sleep session, with REM sleep taking up a larger portion
of the cycle as the night continues. Scientists have advanced several theories
about why humans need REM sleep. One view is that dreaming aids in memory
consolidation. Under this theory, during REM sleep the brain not only strengthens
(25) memory of important actions or events, but it weakens memory of unimportant
things. Another view is that dreaming is necessary for proper central nervous
system development. Yet another theory suggests that REM sleep involves the
repair of monoamine receptors. The theories are not necessarily exclusive; in fact,
they all reach the conclusion that REM sleep deprivation can result in death. The passage supplies information for answering which of the following questions