Students who consider themselves of more than one race are more likely to feel depressed, have trouble sleeping, skip school, smoke and drink alcohol, a study says.

Based on national surveys of 90,000 pupils in middle and high school, the study found that young people of mixed race are at higher risk for stress-related health problems.

For example, students who described their race as both black and white reported more difficulties than those who considered themselves solely black or solely white.

"It did not matter what races the students identified with, the risks were higher for all of them if they did not identify with a single race," said J. Richard Udry, principal author of the study, published in the November issue of the American Journal of Public Health.

While the study suggests that mixed-race teenagers suffer from more stress, Udry said, it does not say why. He noted that many smaller studies have looked at emotional and health issues for teenagers of mixed race.

"The most common explanation for the high-risk status is the struggle with identity formation, leading to lack of self-esteem, social isolation and problems of family dynamics in biracial households," said Udry, a professor of maternal and child health at the University of North Carolina School of Public Health.

Raul Caetano, a professor of epidemiology at the University of Texas School of Public Health, said further research is needed to confirm that being of mixed race causes stress.

"It may be especially important with adolescents, at this developmental stage where the development of your own identity is so important -- it's exactly what you're struggling with," Caetano said. "It's already stressful, and these kids may have an additional layer of stress to deal with."

The findings were drawn from data compiled in 1994 and 1995 by the National Longitudinal Study of Adolescent Health, which is based at the University of North Carolina. Students could give more than one answer when asked their race.

Unlike census data, which adults answer for their households, the survey directly questioned teenagers, who sometimes identify themselves differently than their parents would. This gives a better picture of how teenagers' self-concept affects their health, the authors said.

But some students were inconsistent, giving different answers when asked about their race at different times.

"An adolescent may have problems with ethnic identity the parent may not be aware of or may minimize," Caetano said.

The study found that mixed-race students, compared with single-race students who share part of their racial makeup, were more likely to report having sex at younger ages, having access to guns, getting drunk, considering suicide, and suffering various aches and pains.

Yet in other types of characteristics -- including grades, verbal ability and parents' education -- the study found that mixed-race students tend to fall between the single-race adolescents who shared part of their background. For example, Asian students had higher grade-point averages than whites, and children with both Asian and white parents had averages between those two peer groups.