A research has suggested that going to bed earlier protects teenagers against depression and suicidal thoughts. A US study of 12 to 18-year-olds found those with bedtimes after midnight were 24 percent more likely to have depression than those who went to bed before 10 p.m. The journal Sleep reported those who slept fewer than five hours a night had a 71 per cent higher risk of depression than those who slept eight hours. The study said depression and suicidal thoughts were also more likely in girls, older teenagers and in those who had a lower self-perception of how much parents care about them. <br/> <br/>The researchers from Columbia University Medical Center in New York looked at data from 15,500 teenagers collected in the 1990s. One in 15 of those studied were found to have depression.