Preschoolers who habitually get less than the recommended amount of sleep each night may have difficulty when they enter school, new research suggests.

In a study that followed nearly 1,500 Canadian children from birth to age 6, researchers found that those who slept less than 10 hours per night in their preschool years were more likely to show problems with their verbal and spatial skills by school-age.

These children also tended to be more hyperactive and impulsive than their peers who logged more hours of sleep.

The findings, say the study authors, highlight the importance of young children getting at least 10 hours of sleep each night.

“Our results indicate that a modest but chronic reduction of just one hour of sleep nightly in early childhood can be associated with the child’s cognitive performance at school entry,” the researchers report in the journal Sleep.

Dr. Jacques Y. Montplaisir of the University of Montreal and the Sleep Disorders Center at Sacre-Coeur Hospital led the study.

Experts recommend that preschool-age children sleep 11 to 13 hours per night, while older children should get 10 to 11 hours. But research has suggested that children’s average sleep time has been gradually shrinking over the years.

The issue is important because studies have shown that early-life sleep is vital in children’s intellectual and behavioral development.

For their study, Montplaisir and colleagues followed 1,492 children born between 1997 and 1998. Each year, mothers reported on their children’s typical sleep patterns, from the time the children were 2.5 years old until they were 6.

While most of the children consistently got at least 10 or 11 hours of sleep, the study found, 6 percent habitually slept fewer than 10 hours over the study period. Another 5 percent got fewer than 10 hours of sleep at the age of 2.5, but gradually increased their nightly sleep time as they got older.

Overall, the researchers found, children who were persistently short sleepers were three times more likely to score poorly on a standard test of language skills than children who consistently slept at least 10 hours.

What’s more, children who increased their sleep time as they got older remained at risk of problems - including hyperactivity and poor scores on a test of visual and spatial skills.

This, according to Montplaisir’s team, suggests that there is a “crucial period” in early childhood where lack of sleep is especially detrimental to development, even if a child sleeps more at an older age.

The findings underscore the importance of having children sleep at least 10 hours a night, the researchers conclude, particularly before the age of 3.5.

SOURCE: Sleep, September 1, 2007.