Teenagers who spend more time watching TV, using iPads and playing electronic games take longer to fall asleep and get poorer quality rest. A new study suggests the growth in use of electronic media devices by children could harm their physical and mental health.

The 16-19-year-olds were asked how much screen time they spent outside school hours, and on what activities, for any of six electronic devices: computer, tablet, smartphone, MP3 player, games console（游戏机）and TV. They were also asked questions about their sleep routine on weekdays and at weekends: when they normally went to bed and got up; how much sleep they needed to feel rested; and how long it took them to get to sleep. Girls spent longer on their computers chatting online while boys spent longer playing games on computers.

The study found poorer quality sleep was related to more time spent on such devices, and it often took more than an hour to drop off. Total daytime screen use of more than four hours was related to a 49 percent greater risk of taking longer than 60 minutes to fall asleep.

On average, the teens said they needed 8-9 hours of sleep to feel rested. But those who spent more than two hours emailing or chatting online were more than three times as likely to sleep for less than five hours. Those spending more than four hours in front of any screen were 3.5 times as likely to sleep for less than five hours. Computers were the device most likely to cause problems.

Dr Mari Hysing, of Uni Research Health, Bergen, said screen use may simply replace sleeping time or it may affect sleep by stimulating the nervous system. It is also possible that the light from electronic devices may affect a teenager’s body clock. “The recommendation is not to have a TV in the bedroom,” added Dr Mari Hysing.

Do you think you got enough sleep this past week? Can you recall the last time you woke up without an alarm clock feeling refreshed, not needing caffeine ? 【小题1】. Two-thirds of adults throughout all developed nations fail to obtain the recommended eight hours of nightly sleep.

【小题2】. Routinely, sleeping less than six or seven hours a night destroys your immune system, more than doubling your risk of cancer. Insufficient sleep is a key lifestyle factor determining whether or not you will develop Alzheimer’ s disease. Inadequate sleep, even moderate reductions for just one week, disrupts blood sugar levels so profoundly that you would be classified as pre-diabetic.

Perhaps you have also noticed a desire to eat more when you’re tired? 【小题3】. Too little sleep increases- concentrations of a hormone that makes you feel hungry while stopping a companion hormone that otherwise signals food satisfaction. Despite being full, you still want to eat more. It's a proven recipe for weight gain in sleep-deficient adults and children alike. Worse, 【小题4】, it is in vain, since most of the weight you lose will come from lean body mass(丢脂体重), not fat.

Add the above health consequences up, and a proven link becomes easier to accept: the shorter your sleep, the shorter your life will be. The old maxim “I’ll sleep when I m dead” is therefore unfortunate. Adopt this mindset, and you will be dead sooner and the quality of that (shorter) life will be worse. 【小题5】.

There’s more evidence that what’s good for your heart is good for your head. A new study shows that people who run, swim or do other moderate intensity(强度)exercise have brains that look, on average, 10 years younger than the brains of couch potatoes.

“Our study showed that for older people, getting moderate intensity exercise may be protective, helping them keep their brains work better, ”said Dr. Clinton Wright of the University of Miami, who led the study. But it’s not necessarily easy. Walking, golf, bowling and yoga didn’t count, and people need to start before they begin showing memory loss, Wright’s team reported in the journal Neurology.

The study of nearly 900 people who exercise regularly showed that 90 percent fell into the low-intensity group. These people are part of a larger group taking part in a bigger study called Northern Manhattan Study. They were asked how long and how often they exercised during the past two weeks. Five years later, they were tested for memory and thinking skills and got a brain MRI. Seven years after that, they took the memory and thinking tests again. The 10 percent who said they took part in moderate intensity exercise scored better on the tests. These included running, climbing, swimming, riding bicycles and so on．

“We found that those with moderate exercise had higher scores and slower decline than inactive patients when comparing the results,” the study team wrote.

Many studies have shown that exercise may not prevent Alzheimer’s but may delay it. The findings fit in with a study that found two years of exercising, eating healthier food and brain training can promote people’s memory function.