It isn't only that young people are kept awake by messaging their friends or using the internet. The light from the screen, held close to the face, is physically disruptive to the natural onset of sleep.
From BBC News: Lack of sleep blights pupils' education.
I hear of a lot of people who complain about not being able to sleep and then talk about how when they can't sleep, they stick a bright, flashing, animated light up close to their face as if that's going to help.
As Dr. Karrie Fitzpatrick is quoted as saying in the article:
Having a computer screen that is eight inches away from your face is going to expose you to a lot more light than watching a television on the opposite side of the room.