These different chronotypes are partly explained by genetic variations, partly by habit and behavior, and the underlying biology can be traced in the timing of hormonal ebb and flow, particularly the timing of surges in melatonin, the so-called sleep hormone.
You can assess your own chronotype without doing blood tests with a morningness-eveningness scale questionnaire, which has also been adapted for children and adolescents. That questionnaire was also used in the new study, published recently in Pediatrics.
For the study, researchers asked more than 2,000 7th to 12th graders in the Fairfax County, Va., public schools about sleep and daytime alertness or sleepiness, as well as things like task completion and planning. They were also asked about a range of emotional and behavioral issues.
They found that shorter sleep duration was not directly related to problems in areas ranging from self-discipline to social interactions (as it has been in other studies). Rather, greater daytime sleepiness or having the evening chronotype were much more important for a child’s emotional and behavioral well-being.
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Other studies have shown that the eveningness chronotype is associated in adolescents with various kinds of risk-taking behaviors, from drug and alcohol use to sexual and even financial risk-taking. In adults, the “night owl” phenotype has also been found to be associated with health risks ranging from strokes and diabetes to mood disorders.
“In the last 10 years we’ve come to recognize that there are circadian clocks or oscillators in every cell in the body, and if you’re misaligned with your circadian clock, that’s going to affect all kinds of functions,” said Dr. Judith Owens, the director of sleep medicine at Boston Children’s Hospital and the lead author of the new study. “That’s one of the reasons you feel so lousy when you’re jet-lagged, you have all kinds of physical complaints.”
She and her colleagues suggest in their article that a major issue for the evening chronotype adolescents, and a possible source of problems, is not just sleep deprivation (though these are likely to be the more sleep-deprived adolescents, given the exigencies of early-start high school days) but also that they are not at their most functional and alert biologically when demands are made, from driving themselves to school to taking tests.
“To me, the biggest practical take home message is, it’s not only how much you sleep, it’s when you sleep and how impaired you are by sleepiness,” Dr. Owens said. “It’s like requiring an adult to get up five days a week at 3 a.m. — that’s their circadian trough — for adolescents to be expected to get up and function at a time when their circadian-driven alertness is at its lowest point in 24 hours.” She called moving high school start times later in the morning a critical issue. According to the Centers for Disease Control and Prevention, only 17.7 percent of American public schools start at 8:30 a.m. or later, and getting to school before 8:30 can mean a very early morning.
Monique K. LeBourgeois, a psychologist in the department of integrative physiology at the University of Colorado at Boulder, is interested in the individual variations in chronotype that begin well back before adolescence. In 2014, she and her colleagues published a study in which they measured melatonin levels in 48 healthy children ages 30 to 36 months. None of the children were considered evening types, but there were definite variations in how extremely “morning” they were — and those differences correlated with the time of melatonin release. In other words, even young children vary significantly in their circadian physiology.
So understanding the circadian clock and its variations isn’t just important for sleep problems, Dr. LeBourgeois said in an email. “A mismatch between the timing of the clock and the demands that kids face (or even opportunities for learning and fun) could also result in behavior/emotional problems.”
To recognize individual variations in the master clock is not to say that it is unalterably set for life; the clock is affected by exposure to light, especially blue spectrum light, the kind that comes from LED lights and screens. In recent research linking cellphone and tablet use and poor sleep in children, even having a turned-off device in the bedroom meant poorer sleep. That’s why limiting — and scheduling — light exposure and screen exposure play major roles when you’re trying to reset the clock, for example with adolescents whose cycles have gotten so out of sync that they are suffering from “social jet lag.”
Parents can work with pediatricians to help adolescents move their bedtimes earlier, limiting light exposure and screen exposure in the evening and making sure there is light exposure in the morning.
Very extreme “night owls,” kids who cannot sleep until three or four in the morning, may have what is called delayed sleep phase disorder, Dr. Owens said, and children with this diagnosis need help from sleep experts, which can involve melatonin and light box therapy, along with manipulation of the sleep-wake schedule. Even after their cycles have been reset, they may find themselves slipping back, unless they’re vigilant.
But even if a child’s sleep is more in sync with what the world expects, that child still may be expected to perform in school at the least alert times of day. Getting more sleep will help, but it won’t completely revise that underlying physiology, and parents need to help adolescent children map out their most effective, most alert times for studying, concentrating and learning. It’s worth thinking not just about good sleepers, at any given age, but also about when your best waking function falls in your daily cycle, and how you can play to your strengths.
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