Kids who don't get enough sleep can have problems in school. But there aren't much data on how much sleep children get. To help fill that gap, researchers in Cleveland studied local children.

What the researchers wanted to know: How much sleep do children ages 8 to 11 get every night, and are there differences between boys and girls or among different racial and ethnic groups?

What they did: The researchers studied 887 children who were born in Cleveland from January 1988 to May 1993. The sample was designed to have a lot of premature and minority children so researchers could compare different groups. For seven days, each child filled out a sleep journal that had a form to fill in, for example, if they were sick that day and missed school. In the morning, the children wrote down what time they went to bed, when they tried to go to sleep, how many times they woke up overnight, and what time they woke up in the morning. (Parents were supposed to help fill out the forms if necessary.)

What they found: On average, kids in the study slept 9 to 10 hours a night, which is what's recommended for this age group. But about 15 percent of the children in the study slept less than 9 hours a night. That rate was higher among some children: 43 percent of minority boys ages 10 or 11 slept less than 9 hours a night. Minority children were also more likely to go to bed after 11 p.m.

What the study means to you: Kids who are sleepy during the day are likely to do poorly in school and to have behavioral problems, according to earlier studies.

Caveats: One hundred and thirty-two kids' journals had to be left out for being incomplete. If those kids had more irregular sleep patterns (which is plausible), that would have biased the data. Also, 70 percent of those children were minorities. And, finally, the researchers didn't have enough information on socioeconomic status to give detail to this picture—the differences between white kids and minority kids probably weren't actually because of race but because of socioeconomic status.

Find out more: The Garfield Star Sleeper website, run by the National Heart, Lung, and Blood Institute, has games to teach kids about sleep.

KidsHealth.org gives some guidance on how much sleep your kids need.

Read the article: Spilsbury, J.C., et al. "Sleep Behavior in an Urban U.S. Sample of School-Aged Children." Archives of Pediatrics and Adolescent Medicine. October 2004, Vol. 158, pp. 988–994.

Abstract online: http://archpedi.ama-assn.org