[单项选择] What’’s Happening While Sleeping Sleep is very important to humans. The average person spends 220,000 hours of a lifetime sleeping. Until about thirty years ago, no one knew much about sleep. Then doctors and scientists began doing research in sleep laboratories. They have learned a great deal by studying people as they sleep, but there is still much they don’’t understand. Scientists study the body characteristics that change during sleep, such as body temperature, brain waves, blood pressure, breathing, and heartbeat. They also study rapid eye movement (REM). These scientists have learned that there is a kind of sleep with REM and another kind with no rapid eye movement (NREM). NREM is divided into three stages. In stage one, when you start to go to sleep, you have a pleasant floating feeling. A sudden noise can wake you up. In stage two, you sleep more deeply, and a noise will probably not wake you. In stage three, which you reach in less than
A. Y
B. N
C. NG