The natural shift toward a later sleep/wake cycle that occurs during adolescence — the reason why so many teens have trouble falling asleep before 11 p.m. or so — continues into early adulthood, according to research published in the journal Sleep Medicine.
The study — a large project that tracked the sleep patterns of more than 17,000 young people around the world — also found that the timing and duration of sleep vary by gender and (slightly) by geographical location.
“It was interesting to find that the circadian rhythm shifts later even in people over 20 years of age,” says Liisa Kuula, the study’s lead author and a postdoctoral researcher at the University of Helsinki, in a released statement.
“Geographical differences were relative small but similar to those seen in prior, smaller-scale studies,” she adds. “The need for sleep does not vary greatly between cultures, but differences arise in terms of the time reserved for sleeping.”
These findings have important health implications. Although experts recommend that teens should sleep eight to 10 hours each night for optimal physical and cognitive well-being, less than half of 11- to 18-year-olds meet that goal.
A natural shift
Scientists have long known that children’s sleep patterns shift as they enter their teenage years. That’s primarily because the hormonal changes that occur with puberty also affect young people’s circadian rhythms, the internal “clocks” that set the timing of the body’s physiological functions — including the daily sleep/wake cycle.
Before puberty, the circadian rhythms of most school-aged children cause them to grow sleepy around 8:00 or 9:00 p.m. Once puberty begins, however, those rhythms get pushed back a couple hours later, which is why it’s so difficult for teens to fall asleep until around 10 or 11 p.m.
This natural shift in the sleep/wake cycle during adolescence is called the “sleep phase delay.”
And, yes, environmental factors — such as drinking caffeinated beverages and using television, smartphones or other electronic media in the bedroom — also influence the changes in sleep patterns that occur during adolescence. But it’s the underlying biological changes that drive the sleep phase delay.
For the current study Kuula and her colleagues decided to take a closer look at the duration and timing of sleep patterns during late adolescence and early adulthood.
They used data collected from 17,355 young people aged 16 to 30 who had consented to share information from their electronic fitness trackers with the researchers for a two-week period in early 2018. The participants lived in 107 countries, and more than half (56 percent) were women.
The particular fitness trackers used in the study (three different models of Polar devices) have been found to be reliable for determining when people fall asleep and wake up.
Almost a quarter of a million nights of sleep data were collected. When analyzed, that data revealed the following key findings:
- Sleep duration declines throughout adolescence — and well into early adulthood, stabilizing only around age 30. People in their late 20s sleep, on average, about a half an hour of less per night than those in their mid-teens.
- The delay in the onset of sleep that begins in adolescence continues into early adulthood, peaking around age 22. After that age, people tend to be able to fall asleep at earlier and earlier times. “These findings suggest, write the researchers, “that at least from a sleep timing perspective the transition from adolescence to adulthood may occur in the early 20s.”
- Young women tend to sleep longer than young men — an average of 21 minutes longer per night. They also tend to fall asleep slightly earlier.
The study also uncovered some noticeable regional differences in sleep patterns. Young people in Asia slept the least (6 hours 30 minutes, on average), while those in Australia, New Zealand and other Oceania countries slept the longest (7 hours 14 minutes).
Kuula and her colleagues say that “greater emphasis on work and academic performance in Asian countries” may explain why young people in that area of the world have the shortest sleep patterns.
Young people in Asia, however, also tend to engage in more “catch-up” sleep than those living elsewhere, the researchers point out.
Interestingly, the study did not find that young people slept significantly longer on the weekends than they did on weekdays.
Limitations and implications
The study has several important limitations, particularly the possibility of selection bias. People who wear fitness trackers may not be representative of other people their age. For one thing, they are likelier to lead healthier lifestyles.
The researchers were also unable to include daytime napping in their analysis. The prevalence of napping varies by region and culture, and may therefore explain some of the regional differences in sleep patterns observed in the study.
Still, the study’s findings are interesting — and a reminder that we need to be doing more to help our teenagers and young adults get enough sleep. That means recognizing that young people who are having difficulty getting out of bed in the morning aren’t being lazy or inconsiderate.
Their internal clocks are just in a different time zone.