For the majority, it's simply a stage that passes. But at least 30 percent of children in this age group have difficulties sleeping six consecutive hours – either because they can't fall into slumber or they can't stay asleep. While the effects of lack of sleep on learning are well documented, researchers at the Université de Montréal have found sleepless children can become overweight and hyperactive.

Jacques Montplaisir, a professor in the Department of Psychiatry and director of Sleep Disorders Center at Sacré-Coeur Hospital said that 26 percent of children that sleep fewer than 10 hours a night between two and half years and six years are overweight. The figure drops to 15 percent of those that sleep 10 hours and falls to 10 percent among those that sleep 11 hours.

The research team analyzed a sample of 1,138 children and found: 26 percent of kids who didn't sleep enough were overweight, 18.5 percent carried extra weight, while 7.4 percent were obese.

The relationship between sleep and weight could be explained by a change in the secretion of hormones that's brought on by lack of sleep. "When we sleep less, our stomach secretes more of the hormone that stimulates appetite," Montplaisir explains. "And we also produce less of the hormone whose function is to reduce the intake of food."

Naps don't compensate for nightly lack of sleep, Montplaisir pointed out. According to the same study, inadequate sleep could also lead to hyperactivity. Twenty-two percent of children who slept fewer than 10 hours at age two and a half suffered hyperactivity at six years old, which is twice the rate seen in those who slept 10 to 11 hours per night.

Is it possible that hyperactive children sleep less or that under-slept children become hyperactive? According to Montplaisir, the second scenario is correct. "In adults, inadequate sleep translates into sleepiness, but in children it creates excitement," he says.

Children were also given a cognitive performance test in which they had to copy a picture using blocks of two colours. Among the children who lacked sleep, 41 percent did poorly, whereas only 17 to 21 percent of children with 10 or 11 hours of sleep per night performed badly.

Problems experienced in childhood risk continuing into later years if nothing is done and Montplaisir suggests a new specialty in which sleep problems can be nipped in the bud.

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Materials provided by University of Montreal. Note: Content may be edited for style and length.