A、To start the school earlier. B、To improve the lives of children. C、To stop children from using cell phones. D、To put back scho

Speaking at the British Science Festival on Tuesday, Prof Kelley called for an end to early starts at schools, colleges and universities to "improve the lives of a generation".
    School start times should be put back to as late as 11am to combat a lack-of-sleep crisis among young people, Paul Kelley, the scientist at the University of Oxford has suggested. He said young people in Britain were losing on average 10 hours’ sleep a week, making them lose more sleep than a junior doctor on a 24-hour shift.
    He said children aged 8 to 10 should start school at 8:30am or later, and 16-year-olds should start at 10am and 18-year-olds at 11am.
    Kelley has been working to push for substantial change in the approach to sleep for children.
    "At the age of 10 you get up and go to school and it fits in with our nine-to-five lifestyle," Kelley said. Ignoring our natural body clock could lead to exhaustion, frustration, anxiety, weight gain and high blood pressure, he said, and could make a person more prone to alcohol use and risk-taking.
    The recommendations arise from a deeper understanding of our internal body clock, which determines optimum levels of concentration, wakefulness and work ability.
    He said families had a key role to play in ensuring children get enough sleep. In school sessions aimed at teaching children how to improve their sleep, Meadows said, 96% of participants said they used a phone or mobile device in the last 30 minutes before sleep. "We’re finding that children have phones or tablets from the age of about 10 or 11. These devices emit light which is similar to the light from the sun and they essentially trick our brains into thinking we should be active, not winding down for sleep, and that interferes with our body clock," he said.
    "This is a huge issue for society," Kelley said. "We are generally a lacking-in-sleep society but the 14-to-24 age group lacks more sleep than any other sector of society. This causes serious threats to health, mood, performance and mental health."
    Similar changes are not seen in children who get eight-and-a-half hours sleep a night. Kelley said every governing body of every school in the UK had the power to alter start times if they wish.
20. What did Professor Paul Kelley suggest to combat a lack-of-sleep crisis?
21. What determines the levels of concentration, wakefulness and work ability?
22. Which age group lacks more sleep than others in the British society?