阅读理解：
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 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 studied, including dogs, monkeys, cats, and elephants, and also some birds and reptiles . 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.
However, the dreamer does have control over one type of dream. This type of dream is called a lucid 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.
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