New research shows children should not watch TV, tablet or mobile phone screens before bedtime. Researchers found that "screen time" before sleeping is bad for children's health. The researchers looked at over 60 different studies on how bad screen time is for children aged from 5 to 17. They said that 90 percent of the studies found that screen time delays bedtime. It also leads to fewer hours of sleep, and poorer sleep quality. Screens are becoming smaller, so children can look at them in bed. Most children in the study had some kind of screens in their bedrooms. Children need more sleep than adults. Pre-school children need up to 13 hours, pre-teens up to 12 hours, and teenagers should get around nine hours a night. The researchers found three reasons why screens affected children's sleep. First, the light from screens upsets a child's body clock. It gets into children's eyes before they sleep, and makes children think it is daytime. Second, children watch videos of things that excite them. This keeps their brain active, so they take longer to sleep. Finally, when children are watching screens, they are not exercising, so they are not tired.