People spend about a third of their lives sleeping. That might sound like time is wasted, but it's not. Even in a deep sleep, the brain is busy. It's hard at work sorting(整理) memories, cleansing itself of wastes and more. The time of rest also helps the body become stronger.

That's why experts suggest that teenagers get about nine hours of sleep each night. But surveys(调查) show that teenagers are getting less and less sleep. That's a problem. It will make them feel tired and upset during the day and have difficulty dealing with stress.

It's hard to say exactly why teenagers have become more sleep-deprived(睡眠不足的). One reason may be school start times. As people go through puberty(青春期), they become natural night owls. Their bodies don't produce the hormone melatonin — which tells the brain it's time for bed — until later at night. Sleep usually comes most easily between 11 p.m. and 8 a.m. But most public middle and high schools start before 8 a.m.

Another reason may be screens. A survey about 370,000 American teenagers has shown an increase in trading(交换) sleep for screen time. What's more, the light from phones, computers and other electronic devices makes the brain stop them from sleeping. That's because the light has blue wavelengths also found in sunlight. This cheats the brain and makes it think it's daytime.

Research suggests that people who use screens in bed have a harder time going to sleep. New LED technology could reduce(降低) that influence. But in the meantime(与此同时), experts suggest putting away phones at least 30 minutes before bed. It might be hard to stop thinking about a lively group talk or a wonderful TV show, but turn off the phone and have a good sleep. Your body and brain will thank you.