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Bedtime can be challenging in the summer, with the late evening sunlight streaming through the twins' bedroom window, signaling to them that they should be outside running around instead of tucked under their blankets. 

My wife and I put our 5-year-old twins to bed by 7:30 most nights. This isn't out of any virtuous concern for their health or well-being, mind you, it's primarily because by that point we've had about as much of them as we can take for the day, and need the rest of the evening to unwind and decompress.

But bedtime can be challenging in the summer, with the late evening sunlight streaming through the twins' bedroom window, signaling to them that they should be outside running around instead of tucked under their blankets. Recently it's been enough to make me wonder whether we're putting them to bed too early. What's a "normal" bedtime for a 5-year-old, anyway?

To find out, I got in touch with Dr. Mark DeBoer, a pediatric endocrinologist at the University of Virginia who's done research on the sleep habits of American 5-year-olds. DeBoer pulled data from the Early Childhood Longitudinal Study, Birth Cohort (ECLS-B), a nationally representative study that tracked the growth and development of 14,000 kids born in 2001. When those children were kindergartners, researchers asked their parents what time they went to bed on weeknights.

By DeBoer's calculations, the median bedtime was 8:30 p.m. Half of the kids went to bed earlier, with 40% dropping off between 8 and 8:30 most nights, and the earliest 10% in bed by 8 p.m.

Conversely, 40% of the kids went to bed between 8:30 and 9:30 p.m., and 10% typically stayed up past 9:30.

The relevance is tied to the large body of research that links childhood sleep with various health and educational outcomes. Among kids, insufficient sleep is linked to obesity, poor academic achievement, depression, physical injuries and, as every parent knows, grumpiness. The American Academy of Pediatrics' recommends kids ages 3 to 5 get 10 to 13 hours of sleep each day.

DeBoer has found that a lot of children aren't getting enough sleep, and late bedtimes are a key culprit. "Later bed times are the major cause of short sleep duration," he said via email, driven predominantly by children getting more screen time and parents who've adopted later bedtimes than was seen in previous generations.

His research found, for instance, that kindergartners in the mid-2000s got about 30 minutes less sleep, on average, than those born in the 1970s. That shift was largely driven by later bedtimes; wake times haven't changed much over the years, primarily because schools generally start at the same time they always have.

DeBoer found that television time is a big factor, with kids who watch more than two hours of TV in the evenings going to bed about 20 minutes later than kids who watch less than that.

Overall, DeBoer says, most younger children are at the low end of the recommended daily amount of sleep, with about 1 in 5 not even meeting that threshold. He'd like to see parents make an effort to get their kids to bed earlier. "Parents should have defined bedtimes on the early side for their school-aged children and adhere to these bedtimes," he said.

Regardless, simply knowing what time other kids go to bed could be useful for parents trying to assess whether their own kids are getting enough sleep. Parents of late sleepers may even have some luck convincing their kids to go to bed earlier with the knowledge that most of America's kindergartners are hitting the hay before they are.

Conversely, I won't be telling my own 5-year-olds that most kids their age get to stay up later than they do. And I won't be losing any sleep over it, either.