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 author mentions that the Ganges River dolphin species is blind and uses clicks to send out sonar pulses for guidance and to find food (lines 9-10) most probably in order to
A． support the claim that an underwater acoustic device is helpful in identifying and tracking the Ganges River dolphin B． suggest that, because of this blindness, the Ganges River dolphin survives only in rivers and lakes C． challenge the claim that the species’ decline in numbers is due to fishing net entanglement, human predation, pollution, or construction D． refute the claim that the Ganges River dolphin can in fact be identified and tracked by underwater acoustic devices E． lend credibility to the claim that the Ganges River dolphin is a rare species