Adolescents whose parents enforced bedtimes of 10 p.m. or earlier were significantly less likely to be depressed and to have suicidal thoughts than their peers whose parents allowed them to go to bed at midnight or later, Dr. James E. Gangwisch of Columbia University Medical Center in New York City and his colleagues found.

"It's kind of a common idea that older adolescents don't need as much sleep as younger adolescents, but that's really not true--they still need about 9 hours of sleep at night," Gangwisch told Reuters Health.

Short sleep times and depression have been linked in both teens and adults, he and his colleagues note in their report, and this relationship could be "bidirectional"-meaning getting too little sleep boosts depression risk, while being depressed makes it harder to sleep.

Gangwisch and his colleagues looked at a nationally representative group of more than 15,000 seventh- through twelfth-graders surveyed in 1994-1996. Fifty-four percent of parents said their son or daughter had to go to bed at 10 p.m. or earlier on school nights. Another 21 percent said their child's bedtime was 11 p.m., while 25 percent allowed their children to go to bed at midnight or later.

More than two-thirds of the adolescents said they went to bed when they were supposed to.

Given that parents who were stricter about bedtime might have other qualities that could protect their child from depression, the researchers asked adolescent study participants how much their parents cared for them, and accounted for this in their analysis of the relationship between sleep duration and depression risk.

There was no link between whether a teen had a set bedtime and how much they felt their parents cared for them, but there was a strong relationship between bedtime and whether or not that teen felt he or she was getting enough sleep. And kids with bedtimes at midnight or later were 24 percent more likely to be depressed, and 20 percent more likely to have suicidal thoughts than teens who had to hit the hay at 10 p.m. or earlier.

Kids who got five hours of sleep nightly or less were 71 more likely to be depressed, and 48 percent more likely to have suicidal thoughts, than their peers who got at least eight hours of sleep a night.

And study participants who said they felt they usually got enough sleep were 65 percent less likely to be depressed and 29 percent likely to have suicidal thoughts than those who didn't feel they were sleeping enough.

"Getting adequate sleep is really important for our mental health as well as being able to focus and have the necessary energy and motivation to do the things we need to do during the day," Gangwisch noted.

Getting adolescents to go to bed at a reasonable hour is no easy task, he added, especially given the temptation to text and surf the Internet into the wee hours of the morning. "It's a tall order, especially with adolescents, they kind of have a mind of their own, and they should."

Nevertheless, he added, just asking a teen to give going to bed early try for a few days may convince them, once they see how much better they feel after getting enough sleep.