It may not be news to you that today’s kids are deprived of sleep, but before you blame smartphones or computer games, consider this: children haven’t been getting enough shut-eye for more than a century.

In a new study about sleep deprivation in children, Lisa Anne Matricciani from the University of South Australia in Adelaide and her colleagues reviewed information from previous studies done between 1897 and 2009.

“We were surprised that over the last century, the actual amount of sleep that children are getting was consistently about 37 minutes less than what was recommended for them,” Matricciani said. “The rationale for sleep recommendations was also strikingly consistent for more than 100 years — that children were overtaxed by the stimulation of modern living.”

But while the ramifications of too little sleep are known — poor academic performance, a higher likelihood of obesity, an increased risk of drug and alcohol use and more frequent injuries — she said what her research team found most noteworthy was a lack of solid evidence to support the sleep recommendations made for children.

Timothy Olds, a professor of health sciences at the University of South Australia, said, “This is not to say that kids don’t, in fact, need more sleep, just that the evidence is not out there,” adding how much sleep we think little ones should get may be based more on opinion than fact.

So how much sleep do children need? Dr. Marc Weissbluth, an expert on childhood sleep problems and a professor of pediatrics at Northwestern University Feinberg School of Medicine in Chicago, says to “watch your child, and not the clock.”

Near the end of the day (between 4 and 5 p.m. for a napping child under 3, and between 5 and 6 p.m. for older kids), he advises evaluating mood, personality and performance. If your child is short-fused, clingy or irritable, you may need to adjust naps or bedtimes.

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