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how much   according to   as much as possible   turn off   less than

    In today's world, sleep experts often say that for a better night's rest, Americans shouldtheir smartphones or iPads before sleeping. Such electronic products make the brain more active, and then influence their sleep. Buta latest survey, more than 90% of Americans often use computers or some other kinds of electronic products in the hour before bed.

Researchers find that man-made light from some electronic products at night may reduce (减少) the brain chemicals that help to sleep. In the study, the researchers let volunteers read, play games and watch movies on a screen for different periods of time while measuringlight their eyes received.

They found that two hours of sitting before a bright screen at night reduced the sleep chemical levels by about 22%.

Then the researchers studied the college students who often used computers at night. They got the similar results. And researchers say that may not only cause sleeplessness, but also 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 computerbefore, or turn down our screens before sleeping.