Feelings of depression and low self-esteem plague children as they advance through middle school because they get increasingly less sleep, according to a new study.

"Sleep clearly played a significant role in predicting depressive symptoms and self-esteem during adolescence," said psychologist Jean Rhodes of the University of Massachusetts.

Attempts to improve the health, quality of life and academic careers of adolescents should consider the importance of a good night's sleep, she said.

"Elevated levels of depression and drops in self-esteem are seen as inevitable hallmarks of adolescence," she says. "Yet these results suggest that such changes are partially linked to a variable -- sleep -- that is largely under individual, parental and even school control."

Researchers asked 2,259 Illinois students about the number of hours they slept each night and what grades they received in school. They also answered questionnaires designed to measure depressive symptoms and assess self-worth.

Rhodes and her colleagues found that students who slept fewer hours in the sixth grade had lower self-esteem, higher levels of depressive symptoms and worse grades than students who got more sleep. During the three years of middle school, they also found a steady decline in the average hours of sleep, which apparently led to declines in self-esteem and grades and a rise in depressive symptoms.

Girls had a harder time than boys in getting enough sleep. They got more sleep than boys as they started middle school, possibly because girls enter puberty earlier, creating a greater need for sleep. Levels of sleep dropped for both boys and girls over time, but the decline was steeper for girls, she says.

On average, boys and girls went to bed at the same time. But girls woke up earlier, which other researchers have attributed to longer morning grooming times or a greater burden of household chores.

The research appears in the January-February issue of Child Development.