When We Are Asleep
Everyone dreams, but some people never recall their dreams, or do so very
rarely. Other people always wake up with vivid recollections (记忆) of their
dreams, though they forget them very quickly. In an average night of eight
hours’ sleep, an average adult will dream for around one hundred minutes,
probably having three to five dreams, each lasting from ten to thirty
minutes. Scientists can detect when someone is having a dream
by using an instrument which measures the electrical waves in the brain. During
dreaming, these waves move more quickly. Breathing and pulse rate also increase,
and there are rapid eye movements under the lids, just as though the dreamer
were really looking at moving objects. These signs of dreaming have been
detected in all mammals (哺乳动物) studied, including dogs, monkeys, cats, and
elephants, and also some birds and reptiles (爬行动物). This period of sleep is
called the "D" state for around 50% of their sleep; the period reduces to around
25% by the age of 10. Dreams take the form of stories, but they
may be strange and with incidents not connected, which make little sense. Dreams
are seldom without people in them and they are usually about people we know. One
estimate says that two-thirds of the "cast" of our dream dramas are friends and
relations. Vision seems an essential part of dreams, except for people blind
from birth. Sound and touch senses are also often aroused, but smell and taste
are not frequently involved. In "normal" dreams, the dreamer may be taking part,
or be only an observer. But he or she cannot control what happens in the
dream. However, the dreamer does have control over one type of
dream. This type of dream is called a "lucid" (清醒的) dream. Not everyone is a
lucid dreamer. Some people are occasional lucid dreamers. Others can dream
lucidly more or less all the time. In a lucid dream, the dreamer knows that he
is dreaming. In a lucid dream we can use Morse code to communicate with others.