In today's world, sleep experts often say that for a better night's rest, Americans should turn o their smart phones or iPads before sleeping. Such electronic products make the brain more (act), and then influence their sleep. But according to a latest survey, more than 90% of Americans often use a computer or some other (kind) of electronic products in the hour before bed.

    Researchers find that man-made light from some electronic products at night may reduce the brain chemicals that help to s. In the study, the researchers let volunteers read, play games and watch movies on a screen for different time while measuring(测量)  much light their eyes received. They found that two hours of (sit) before a bright screen at night reduced the sleep chemical levels by about 22%.

    Then the researchers studied the college students who often (use) computers at night. They got the s results. And researchers say that may not only cause sleeplessness, but  raise the risk of obesity（肥胖症）, diabetes（糖尿病）and other diseases.

    To be on the safe side, the researchers suggest that we should use computer (little) than before, or turn down our screens as much as possible before sleeping.