CHAPPAQUA, N.Y. (The New York Times News Service)- To Donna Swift, it's obvious that some of her third-graders are not getting enough sleep.

Some rest their heads on their desks and others slump low in their seats. When she asks them to sit in a circle in the corner of her classroom at Roaring Brook Elementary School, a few curl up on the rug.

Then there are the 8- and 9-year-olds who begin the day looking tired, only to grow increasingly keyed up. By afternoon they have trouble sitting still and following directions.

"It's really hard to teach a tired kid," Swift said. "You have to prompt them a lot more to get them to stay with you or to focus so they can answer a question. Their focus, their attention span and their enthusiasm is gone. For some, it's like they're literally too tired to care."

Adults, of course, have long been subjects of insomnia studies. And it is hardly news that teen-agers are often up late into the night and nearly impossible to wake in the morning. But until recently, sleep researchers thought that elementary school students were generally sound sleepers.

Yet new studies show that a significant number of children ages 5 to 10 struggle to get a good night's sleep, and many educators seem to be paying attention to the problem. The National Association of Elementary School Principals, in a campaign to underscore the sleep needs of young students, has suggested a bedtime of 9 p.m., noting that sleep-deprived children "can't concentrate on their school work, they have trouble remembering things, and they may become irritable and fidgety."

Studies of adults and children have shown that lack of sleep impairs complex cognitive tasks. A tired child may be able to solve a simple math problem, but may struggle to master a word problem that requires several logical steps. Sleepiness also slows a child's reaction time and his ability to organize thoughts. Inventiveness, too, can be impaired by fatigue.

Lack of sleep can also mimic the symptoms of some attention deficit disorders, and doctors are studying the connection between sleep disorders and learning and behavior problems.

"These relationships are only beginning to be looked at," said Dr. Lauren Broch, director of education and training at the Sleep-Wake Disorders Center at New York Presbyterian Hospital.

Children lose sleep for a number of reasons. For some, it is simply a matter staying up too late, whether they are watching television, going out at night, living in a chaotic household or using their flashlight to read under the covers well into the night. For others, the problem is biological. Doctors say these children suffer from disorders like sleep apnea and restless legs syndrome, which interrupt their rest all night long.

Whatever the cause, it can be difficult to identify sleep-deprived children because they do not always show the same symptoms as tired adults, doctors say. While an adult might appear lethargic and complain of fatigue, a tired child might instead be crawling over desks, jiggling his leg, repeating his questions, careening around the classroom, or becoming so giddy at a joke he can't stop laughing.

"Sleepy kids don't necessarily yawn," said Dr. Rochelle Zak, a neurology instructor at New York Presbyterian Hospital. "A sleepy kid is inattentive and has trouble concentrating and focusing. Sleepy kids may move around a lot. They may appear hyperactive but what they're doing is trying to keep themselves alert and awake."

When it is time for her students to take standardized tests, Mary Reese, the principal of Menlo Park Elementary School in Edison, N.J., sends a letter home reminding parents that the best way they can enhance their children's performance is by ensuring they get a good night's sleep. And when teachers report that a child is chronically tired, Reese tries to follow up with the parents.

"If a child is not getting enough sleep because of homework, we have to adjust that," Reese said, adding that grade school children should have no more than an hour of homework each night. "We also stress the idea of having good routines with kids, including reasonable bedtimes. But these are only things we can suggest. Obviously we don't have the power to implement them."

Dr. Judith A. Owens, who runs a pediatric sleep-disorders clinic at Hasbro Children's Hospital in Providence, R.I., and a clinic for children with attention deficit hyperactivity disorder, noted "a tremendous overlap" in symptoms at the two clinics.

"When kids get sleep-deprived, they get whiny and they have very low frustration tolerance," Owens said. "A second area that is particularly affected is reaction time, ability to focus, level of attention span. With the behavioral problems we get aggression, hyperactivity, impulsiveness. And that starts to look like ADHD in a lot of ways."

In a study of sleep habits of almost 500 grade school children from kindergarten through fourth grade, Owens found that more than a third were identified by their parents as having some sort of sleep problem, running the gamut from resistance at bedtime to bed-wetting and nightmares. And when researchers asked the children themselves about their sleep habits, they reported more difficulty falling asleep and more incidents of waking up during the night than their parents were aware of.

Owens' original study was of a mostly middle class, largely white school population in Nebraska. But she has done further studies of urban, Hispanic and African-American children and found a higher prevalence of sleep problems.

Most grade school children should get about 10 hours of sleep a night, with kindergarteners and first-graders getting closer to 11, doctors say. For children who do not sleep enough, the solution is as straightforward as enforcing earlier bedtimes. But children who suffer from sleep that is fragmented and disrupted should be screened for sleep disorders. Over the last 15 years, researchers have identified three main sleep disorders in children.

One is sleep apnea, in which breathing becomes obstructed during sleep, causing the child to wake up frequently. Sleep apnea in children is caused primarily by large tonsils and adenoids. Other disorders include restless legs syndrome - in which children report uncomfortable sensations in their legs - and periodic limb movement disorder, which causes children to kick in their beds.

In some cases of sleep apnea, removing a child's tonsils and adenoids can cause dramatic improvement in behavior and academic performance. Sometimes children being treated for an attention disorder may be taking a medication like Ritalin, which makes it more difficult to fall asleep. And children who are hyperactive may simply be unable to calm down at bedtime.

"I'm convinced that there's a subgroup of kids with ADHD who just have tremendous problems falling asleep - they just can't turn their brains off," Owens said. "They are just lying in bed awake for an hour, two hours or three hours. In that situation we really don't know how much of the subsequent focus and attention problem is due to the sleep problems, or just exacerbated by them. But the big point is that with every child with a learning problem, you need to look at their sleep."

Copyright 2002 The New York Times News Service. All rights reserved.