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When We Are Asleep

Everyone dreams,but some peopole never recall their dreams,or do so very rarely.Other people always wake up with vivid recollections (記憶) 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 (哺乳動(dòng)物) studied,including dogs,monkeys,cats,and elephants,and also some birds and reptiles (爬行動(dòng)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 are senses 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.

16.Some people dream but cannot remember their dreams.