Going to bed earlier protects teenagers against depression and suicidal thoughts, research suggests.

A US study of 12- to 18-year-olds found those with bedtimes after midnight were 24% more likely to have depression than those who went to bed before 10pm.

And those who slept fewer than five hours a night had a 71% higher risk of depression than those who slept eight hours, the journal Sleep reports.

The researchers from Columbia University Medical Centre 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.

As well as the higher risk of depression, those who were set a bedtime by their parents of after midnight were 20% more likely to think about suicide than those whose bedtime was 10pm or earlier.

Those who had less than five hours sleep a night were thought to have a 48% higher risk of suicidal thoughts compared with those who had eight hours of sleep.

Teenagers who reported they "usually get enough sleep" were 65% less likely to be depressed.

Depression and suicidal thoughts were also more likely in girls, older teenagers and in those who had a lower self-perception of how much their parents cared about them.

Most of the parents of the adolescents in the study set a bedtime of 10pm or earlier.