SATURDAY, July 19 (HealthDayNews) -- Parents may be able to get by on little sleep from time to time, but shortchanging kids on slumber can turn their days into nightmares.

Studies show younger children who don't get enough sleep -- nine hours is considered optimum for elementary school children -- can suffer in school, are at higher risk for accidents and could even be mistakenly thought to have attention-deficit hyperactivity disorder (ADHD) because the symptoms of sleep deprivation closely mirror those of ADHD.

This is not a small problem, says Carl Hunt, director of the National Center on Sleep Disorders Research at the National Heart, Lung, and Blood Institute (NHLBI).

"All the evidence we have suggests that perhaps a third of children don't get the amount of sleep they need," he says, "and the price children pay for not getting enough sleep is an increased risk for accidents and problems in learning, mood and behavior."

Exacerbating the problem, Hunt says, is that the sleepiest children often don't seem sleepy at all.

"A major problem with children if they are sleep-deprived is that once they get up and get going, they don't act sleepy during the day and early evening. Instead, the sleep deprivation may have the opposite effect, and they may be overactive," he says. "Because they don't act sleepy, parents don't appreciate that the cause of their hyperactivity is sleep deprivation, but there is a clear association between lack of sleep and overactivity."

Recent studies confirm the additional risks of sleep deprivation.

In the March/April issue of Child Development, Israeli psychologist Avi Sadeh reported that among a group of elementary school children, as little as one extra hour of sleep a night significantly improved school performance.

Among a group of 77 fourth- and sixth-graders who were tested before the study and again after they had either added or eliminated an hour of sleep, Sadeh found those who had more sleep improved their performance by as much as two grade levels on tests assessing attention span and memory, both of which are necessary for optimum school performance.

Those who lost an hour of their regular sleep showed no improvement on the memory and attention span tests, and on tests measuring reaction times performed significantly poorer than they had before they were sleep-deprived.

"This is the first evidence that really minor variations of sleep in children can have measurable effects on cognitive function," Sadeh says.

The playground can become a more dangerous place for tired children as well.

A recent Italian study of approximately 300 children under 14 who visited an emergency room in Udine, Italy, compared the sleep patterns of children on the days they were injured to their sleep patterns when they weren't hurt and found a link between lack of sleep and increased risk of injury.

Among the findings: Sleeping less than 10 hours a day was associated with an 86 percent increase in risk for injury; and children between the ages of 3 and 5 who slept less than 10 hours a day seemed to have a significant increase of risk of injury, especially boys.

Getting kids to sleep earlier is a challenge, experts agree.

"Insufficient sleep is becoming a major health concern all over the world. But it is a challenging one, especially for children, because the attraction for kids to television, the computer and other media, as well as school demands, are very hard to compete with," Sadeh says.

Add to that the fact that sleep gets no respect.

"Society in general and adults undervalue sleep," Hunt says. "Most people need between eight and 10 hours a night, but people say, 'That sounds good, but I don't need that much sleep.' "

Young children need more sleep, at least nine hours a night on a consistent basis, Hunt says. "So the first step is for parents to appreciate that sleep matters and understand the price children pay for not getting enough sleep," he says.

The NHLBI offers the following tips for parents to help get their kids to bed.

• Set a regular time for bed each night and stick to it.
• Establish a relaxing bedtime routine, such as giving your child a warm bath or reading him or her a story.
• Avoid giving children a big meal close to bedtime, and no caffeine within six hours of sleep.
• Make after-dinner playtime relaxing; too much exercise close to bedtime can keep children awake.
• Keep the noise level down and the bedroom dark. If some light is necessary, use a small nightlight.