Despite kids’ protests, enforcing early bedtimes may be good for their mental health. Teens who are allowed to go to bed later are more likely to suffer from depression—probably for the simple reason that they are not getting enough sleep, a recent study suggests.

Columbia University scientists found that depression was 24 percent more common in teens whose parents let them go to bed at midnight or later than in kids whose moms and dads required them to hit the pillow by 10 P.M. The night owls were also 20 percent more likely to have suicidal thoughts.

Teens with bedtimes of midnight or later got an average of seven and a half hours of sleep, whereas those with a lights-out of 10 P.M. or earlier got an average of eight hours and 10 minutes. Although the association between later bedtimes and depression was greater before controlling for parents’ marital status and poverty level, it remained statistically significant after taking those things into account—as well as teens’ perceptions of how much their parents cared about them.

The researchers looked at parent-enforced bedtimes—as opposed to simply logging hours slept—to rule out the possibility that depression was causing some kids to sleep less, rather than the other way around.

Earlier work supports the idea that too little sleep may lead to depression. Research at the University of London showed that children who suffer from insomnia are at increased risk of developing depression in their tweens and teens. And a University of Pittsburgh study of youth at risk for hereditary depression found that the one biological predictor of resilience—in other words, not getting depressed—was adequate sleep. Although too little sleep is unlikely to be solely responsible for a teen’s low mood, in those with a genetic or environmental predisposition sleep loss may raise risk and satisfying rest may be protective.

Recent studies at Walter Reed Army Medical Center and the University of California, Berkeley, are starting to tease out why. During brain scans, sleep-deprived but otherwise healthy people showed increased activity in the amygdala (the brain’s emotional center) and decreased activity in the prefrontal cortex (an area that puts our experiences in context, and by extension, makes us rational)—the same changes seen in people who are depressed. In one army study, subjects started to show symptoms of depression, and the Berkeley subjects became more distressed than rested participants when confronted with upsetting images.

All these neurobiological effects may hit teens especially hard, says psychologist William D. “Scott” Killgore of Harvard Medical School–affiliated McLean Hospital, a co-author of the army research. As teens cope with increasingly complicated daily life, they need more sleep than younger kids or adults, Killgore explains, and so “not getting enough sleep is especially problematic.”