Friday 阅读理解 D

Most sleep experts agree that teenagers need 9 hours of sleep each night to be engaged and productive students; less than 7 hours is considered to be insufficient （ 不足的 ） sleep. A peek into any sleepy-eyed classroom in the country will tell you that many young people do not have enough sleep, but it's unclear whether young people today are in fact sleeping less.

To find out, Jean M. Twenge and other researchers examined data from two long-running, nationwide, government-funded surveys of more than 360,000 teenagers.

Combining and analyzing data from both surveys, the researchers found that about 40% of teenagers in 2015 slept less than 7 hours a night, which is 58% more than in 1991 and 17% more than in 2009. Studying further the data, the researchers learned that the more time young people reported spending online, the less sleep they got. Teens who spent 5 hours a day online were 50% more likely to not sleep enough than their peers who only spent an hour online each day.

“Teens' sleep began to shorten just as the majority started using smartphones,” said Twenge in his book about teenagers' sleeping. “It's a very suspicious pattern.”

For many, smartphones and tablets are an indispensable part of everyday life, so the key is moderation （适度）. Limiting usage to 2 hours a day should leave enough time for proper sleep. And that's valuable advice for young and old alike.

“Given the importance of sleep for both physical and mental health, both teens and adults should consider whether their smartphone use is interfering with their sleep,” Twenge says. “It's particularly important not to use screen devices right before bed, as they might interfere with falling asleep.”