In today's world, sleep experts say that for a better night's rest, Americans should turn off their smartphones or iPads before sleeping. Such electronic products make the brain more active, and then influence people's sleep. But according to a latest survey, more than 90% of Americans usually use a computer or some other kinds of electronic products in the hour before bed.

Researchers found that man-made light from some electronic products at night might reduce the levels of the brain chemicals that help sleep. The researchers showed that sitting in the light from tablets clearly reduced the levels of the chemicals which manage our body clocks and play a great role in the sleep cycle.

In the study, the researchers let volunteers read, play games and watch movies on a tablet for different time while measuring how much light their eyes received. They found that sitting for two hours before a bright tablet at night reduced the levels of the chemicals by about 22%.

Studies of students using computers at night have got the similar results. And researchers said that might not only cause sleeplessness, but also increase the risk of obesity(肥胖癥), diabetes(糖尿病) and other diseases.

To be on the safe side, the researchers advise us to use computers less before bed, or turn down our screens low.

1.What can we learn from the first paragraph?

A.Americans don't get enough sleep. B.Electronic products are very popular.

C.The brain becomes more active at night. D.Sleep experts never use smartphones.

2.How many electronic products influence people's sleep?

A.By making our brains more tired. B.By making our bodies weak.

C.By reducing the levels of the brain chemicals that help us sleep. D.By changing our body clocks.

3.What does the underlined word "that" refer to?

A.Making a survey about sleep. B.Using smartphones in the daytime.

C.Sitting in a bright room. D.Surfing the Internet at night.

4.What do the researchers advise us to do to have a good sleep?

A.Learn to use smartphones or iPads. B.Watch some relaxing TV programs.

C.Turn down the screens low. D.Try to sleep earlier than before.

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