5．
Have you ever experienced the feeling of falling as you start to fall asleep？Sometimes you may come to realize this feeling．If so，don't worry．Many people have felt the same thing．
In fact，these muscle twitches （肌肉抽搐） in the arms，legs or other parts of the body are called hypnic jerks （入睡抽動） or sleep starts．And they are very common．Up to 70percent of people experience them sometimes，Huffington Post reported．But what causes it？
There are some theories（理論）．Some scientists believe that the feeling of falling comes from the body relaxing itself，especially if the person is anxious or unable to get comfortable．As you're about to sleep，your muscles become relaxed．But your brain stays awake．It misunderstands the relaxing of your muscles as a sign that you're falling．Then the brain jerks the body awake as if it's trying to protect you．
Other scientists believe that different phases of sleep might cause the hypnic jerks．When we sleep，we experience two phases：non-rapid eye movement （NREM，非快速眼動睡眠期） and rapid eye movement （REM，快速眼動睡眠期）．During REM，our heart rate （心率），breathing and nervous system can act irregularly （不規(guī)律地）．If the body experiences flashes （一瞬間） of REM while entering sleep，these irregularities could cause twitches．
If you don't want to experience hypnic jerks，scientists have some suggestions．When you become deeply relaxed，you can learn how to reconnect （再聯(lián)系） slowly by moving your fingers and toes a little bit．Then start to move your head and open your eyes，while being aware of yourself as you get out of the relaxed state．
But experts agree that hypnic jerks，though sometimes scary（嚇人的），are harmless．If it wakes you up，simply roll over and go back to sleep．But keep in mind：a caffeinated （含咖啡因的） drink，exciting evening activities or stress may make hypnic jerks happen more often．
39．We know from the passage thatD．
A．they happen as people wake up
B．muscle twitches are only in the arms and legs
C．they can cause people to fall out of bed
D．more than half of people experience them sometimes
40．Some scientists believe that people usually have the feeling of falling whenA．
A．their body tries to relax itself
B．they feel safe
C．their brain needs to rest
D．they feel comfortable
41．From Paragraph 4，we learn thatC．
A．we experience three phases of sleep
B．hypnic jerks take place during NREM
C．our heart rate acts irregularly during REM
D．our nervous system acts regularly during REM
42．Scientists think thatB．
A．there is no way to avoid hypnic jerks
B．hypnic jerks do no harm to your body
C．you'd better not completely relax while sleeping
D．exciting activities can help hypnic jerks happen less often．
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