The questions that kids ask about science aren’t always easy to answer. Sometimes, their little brains can lead to big places that adults forget to explore. That is what inspired our series Science Question From A Toddler, which uses kids’ curiosity as a jumping-off point to investigate the scientific wonders that adults don’t even think to ask about. The answers are for adults, but they wouldn’t be possible without the wonder that only a child can bring. I want the toddlers in your life to be a part of it! Send me their science questions, and they may serve as the inspiration for a column. And now, our toddler …

Q. Why is it bedtime if it’s still light outside? — Kayla N., age 5

Ah, yes. I remember this scenario well. A warm evening breeze. Neighbors mowing the lawn. Golden light wrestling with venetian blinds. And me, 7 years old, dressed in my jammies and tucked into bed like a chump.

Truly, bedtimes are one of the great injustices of American childhood. Turns out, they’re also a pretty good example of how sleep — a biological need that we can’t live without — is intertwined with the much more subjective vagaries of culture. It’s culture, after all, that convinced my parents that I needed to be in bed by 7:30 p.m. in July. And my still slightly simmering resentment of that fact, while anecdotally pretty normal among my late Gen X/early millennial American peers, might not be universal.

Research on American kids suggests that “bedtime resistance” — that fist-in-the-air struggle against the oppressive forces of lights-out via tantrums, conveniently timed bathroom breaks and the need to ask right now about where babies come from — is common and increases as children age. The New York Longitudinal Study, which lasted from 1956 to 1988, found that 26 percent of 2-year-olds exhibited bedtime resistance behaviors, and that figure rose to 50 percent by the time kids were 5. But kids whose behavior was documented in similar longitudinal research in Switzerland weren’t as rebellious. A 2005 study using that data found that, for them, bedtime resistance peaked between 2 and 4 years old, at around 18 percent. And rates of youthful rebellion changed as parental behavior changed. The 2005 study also found that bedtime resistance had been decreasing over time. The peak for kids born in 1974-78 was about 30 percent prevalence at age 5. Meanwhile, resistance among kids born in 1986-93 peaked at age 3, closer to 10 percent. Over that time period, the authors wrote, Swiss parents had shifted toward later and later bedtimes. In Switzerland, at least, putting kids to bed later meant less frustration for everyone.

Kids need to sleep, and they generally sleep more than adults, especially when they’re really young. But there’s a lot of variability in what’s healthy. The National Sleep Foundation, a nonprofit that funds research and does public outreach on sleep health, says it could be appropriate for a toddler to get as few as nine or as many as 16 hours of sleep a day. What constitutes normal, healthy sleep changes between individuals, and kids are even more variable than adults, said Oskar Jenni, a researcher at the Child Development Center of the University Children’s Hospital Zurich in Switzerland. That fact often clashes with the reality that bedtimes are sociocultural decisions based, at least in part, on parents’ expectations of how long kids should sleep. Scientists are still trying to understand how culture and biology work together to create our sleeping lives, and they are learning that cultural variables can have a big impact on outcomes of sleep that we might be tempted to view as purely biological: how easily we fall asleep, how we feel the next day and even what it means to “get enough.”

Biology is the biggest determinant of sleep, said Kate Bartel, who recently completed her Ph.D. under professor Michael Gradisar at his sleep science laboratory at Australia’s Flinders University. She’s talking specifically about the way the body responds to light, triggering complex cycles of physical and behavioral changes. The suprachiasmatic nucleus, or SCN, a tiny area of the brain near the optic nerves, uses the light picked up by your eyes to calibrate itself as a timekeeping device for your body. Electrochemical impulses produced by the SCN help to control a wide range of bodily functions, including when you get hungry, how much of what you eat turns into fat and whether your skin is ready to protect itself from sun exposure — processes that are commonly referred to as the circadian rhythm.

Those same signals from the SCN tell you when to sleep by triggering the production of melatonin — a hormone that, among other things, makes humans tired — in response to darkness. The sun sets, melatonin floods your system and the couch starts looking pretty nice.

But how we sleep — and how we experience the quality of the sleep we get — isn’t driven purely by biology. There’s a cultural aspect to this as well, and you can see it in some disorders of the circadian rhythm, like delayed sleep-wake phase disorder. People who have this problem find themselves unable to fall asleep until well after midnight, Bartel told me. Some don’t even successfully pass out until after 5 a.m. As a result, they sleep late into the day. The whole timing of their lives shifts. Effectively, their circadian rhythm is different from that of everyone around them.

Sleep-wake phase disorder usually starts in adolescence. There’s probably biology involved in who develops it, but it can also be related to behavior, Bartel said. A kid might start off staying up late to play video games every night and eventually be unable to sleep at a culturally accepted bedtime — even when she wants to. The prevalence of this sort of disorder varies across cultures, and culture is what makes it a disability — school, work, shopping, all of that happens at wildly unworkable times for people with sleep-wake phase disorder. Most will end up getting therapy, Bartel said, and will attempt to change their rhythm with behavioral adjustments, hormones or exposure to light. That works for some people. But not all.

And that’s where the real problems lie. It’s all fun and games when your body’s sleep biology matches up well with what society expects. But when those two things are out of sync — as in the case of a wide-awake child with an early bedtime — things get messy.

Consider, for a moment, what could happen if one culture’s bedtime expectations were treated as the only healthy choice and exported worldwide. In 2005, Jenni published a paper in the Journal of Sleep Research critiquing an earlier paper that tried to do just that — define childhood insomnia as occurring when a 7-year-old can’t fall asleep by 8:45 p.m. In his critique, Jenni pointed out that this definition ignored the fact that average bedtimes varied widely from country to country. For instance, out of six countries whose data Jenni reviewed, three had bedtime norms that would make a perfectly average 7-year-old a candidate for medication.

American high schools are another great example of what happens when biology and cultural expectations clash, said Bonnie O’Connor, professor emerita of pediatrics at Brown University. Research suggests that teenagers have different circadian norms than everybody else — a sort of temporary phase shift that means their bodies are happier staying up later and sleeping in longer. But the average start time for U.S. middle and high schools is 8:03 a.m. The American Academy of Pediatrics and the Centers for Disease Control and Prevention have recommended that schools start no earlier than 8:30 a.m., saying that earlier start times are linked to poor academic performance and a host of social ills, such as drinking and depression.

But it’s not easy to make that change, O’Connor said, even if it really would be better for teens. Even if school starts later, parents’ jobs don’t, which would mean more teenagers who have to get themselves up, fed, dressed and to school without any adult supervision or assistance. Because of the impact on bus schedules, younger students could end up starting school earlier than ever. And the shift would push extracurriculars back later and later, interfering with family time, homework and, ironically, bedtimes. “Making exceptions for adolescents disrupts the hell out of everybody else’s schedule,” O’Connor said. Even a fix doesn’t necessarily make a good fit.

But the impact of culture on sleep isn’t always negative. Bartel told me about some traditional indigenous societies that seem to moderate the impact of insomnia by simply not considering it a big deal. “When people [in Western, industrialized societies] wake up in the night, one of the big issues is that they perceive it to be an issue,” Bartel said. You lie awake thinking about lying awake. If the cultural approach to insomnia is more like “meh, that happens sometimes,” maybe it drains off a little of the power that insomnia has over you.

All of this means that it’s hard to pin down what “normal” sleep looks like or what “ideal” sleep is — a problem exacerbated by the fact that we still don’t have a lot of good data about the impacts of cross-cultural sleep differences, both within countries and between them. For instance, scientists know that teens from many Asian countries, including China and Japan, seem to get less sleep than most of their peers in the U.S. and Europe. But, Jenni said, we don’t know a lot about whether that’s a problem or just a difference. Are kids depressed? Are they suffering academically? Maybe. Maybe not. Less sleep is not necessarily the same thing as “not enough,” Jenni said.

Ultimately, O’Connor said, we have to be cognizant that sleep is a biological phenomenon, but how sleep is done is social and cultural. There’s not one right way to do it that produces a good fit for everybody, everywhere, all the time. And, at least on a family or individual scale, you’re probably best off finding a compromise that gives everybody involved the best possible fit between biology and culture. In other words, maybe there isn’t a good reason that Kayla has to go to bed before it’s dark out. But that’s for her to work out with her parents, once she can read.

This has been a message from Former Children In Support of Later Bedtimes.