Children at boarding school get the best night's sleep because of strict lights out time, according to a recent study, which was presented at Sleep Down Under, the 33rd yearly scientific meeting of the Australasian Sleep Association. Professor Kurt Lushington, a psychologist who led the study; said, "It is a naturalistic experiment showing the key to good sleep in teenagers is quite simple: a good timetable and no mobile devices（设备） at night."

Over the past 20 years, the time children spend sleeping has fallen and only half regularly get more than seven hours. The recommended amount is eight to ten. The biggest problems are smartphones and tablets （平板电脑） whose blue light wakes the brain, making it harder to drop off.

An Australian team compared sleep patterns of 15 to 18-year-olds at a school in Adelaide. The 59 boarders had to leave their phones in a kitchen area or on their desk before bed and turn their lights out by a set time. They slept for an average 8 hour 26 minutes a night compared with 7 hours 46 minutes for day pupils.

Lack of sleep affects mental health and school performance and causes behavioral problems. It has also been connected with the child obesity epidemic and other diseases. This year an American study found that teenagers who didn't get enough sleep took in extra sugar during a school year. They ate sweets, cakes, chocolate and biscuits and drank fizzy drinks to promote their energy levels.

Lushington recommends that parents reach agreement on not using a phone. This is most likely to work if they do likewise and offer other enjoyments, such as books. "I think if you put these rules in place, kids will go along with them," he told New Scientist.