The terms "night owl" and "early bird" have floated around in conversation for ages before scientists developed the jargon "chronotype" to describe a set of somewhat stable behavioral differences among people. Some individuals really are morning people, going to bed earlier and having their peak performance in the morning. Late-night sorts also exist, and there's a spectrum of people somewhere in between. Leading a lifestyle that doesn't match your chronotype leads to what's called a social jet lag, which has been linked to everything from school performance to frequency of car crashes.

But the "somewhat" portion of the "somewhat stable" description of chronotype is very real. People's chronotypes change as they age, and there's some indication that it can adapt to everything from light exposures to lifestyle.

All of that seemingly comes together at a rather important point in people's lives. School tends to start early, which studies have indicated works out well for the grades of morning people. And exacerbating this is the fact that adolescents normally see their chronotype shift ever later, typically reaching a lifetime peak in the late teens. Now, a group of Argentine scientists has tracked what happens to students' performance when there are different mismatches between chronotype and school start times.

A unique resource

As we noted above, there have been some indications that chronotype and school start times affect students' grades. But these studies all suffered from a serious limitation: pretty much everywhere starts school early in the morning. So, what most of these studies are really testing is whether early birds perform better at school. It's entirely possible that an early chronotype is indirectly associated with other mental traits that help improve school performance. While a few studies have found that shifting school starts later improves grades, even a later start time is a bit early for most adolescents, who are likely to be at their peak of late-night preferences. Put differently, studies with students are by necessity done at a time when they're suffering from a large social jet lag.

(Oddly, these studies have shown that effects are subject-specific. A chronotype mismatch causes problems with math and chemistry but doesn't seem to impact language or geography.)

The new study manages to deftly avoid all this by taking advantage of an extremely rare situation. A school in Buenos Aires runs morning, afternoon, and evening classes, and students are randomly assigned to one of these time slots. Thus, the researchers had access to a population of over 750 students who had very large differences in how their school start time might match up with their chronotype. The location also contributed, as Buenos Aires residents have developed a lifestyle where families' evening meals typically occur within a few hours of midnight, potentially exacerbating the mismatches between chronotypes and different school start times.

The study population included members of all three start times (about eight in the morning, noon, and 5pm). It also had two different age groups, one young enough that the shift to later hours hadn't yet become dramatic, and an older group that was right in the thick of it. All of them were surveyed to figure out their chronotype but weren't informed of what the study was examining.

Mornings are the worst

Since the assignment of students was random, you'd expect each group to comprise people with a correspondingly random mix of chronotypes. Instead, the researchers found that the morning group had the earliest chronotype of the three, indicating that the students had managed to adjust to the earlier starts. In the older students, the average chronotypes of the afternoon and evening students was roughly an hour later than that of their younger equivalents based on the measurement used by the researchers. But the students in the morning classes saw it increase by only a matter of minutes, despite the large impact of age on this measure.

By comparing the students' wake-up times on weekends and during the school week, the authors estimated the social jet lag involved in the early start. They found it was close to four hours for everyone with the morning start time. For the older students, even the afternoon classes posed a bit of a problem, as they tended to wake up earlier than they would have in order to take care of other tasks in the morning (tasks that could have included homework; the researchers didn't specify).

As found in other studies, an early start to school was hard on those students who didn't have a correspondingly early chronotype. Each hour of additional mismatch saw math scores drop by 0.32 points (where a minimum passing grade is 26). For all other subjects, the difference was 0.16 points.

But, outside of those circumstances, things get complex. For younger students who don't have large chronotype differences, afternoon classes saw no differences in performance associated with chronotype. For older students, afternoon classes saw no difference in math, but early chronotype students do better in language. By the evening classes, all the chronotypes seemed to perform equally well.

Not entirely an answer

So, what does this tell us about chronotypes? The report does extend previous results by showing that, on average, students benefit when there's a better match between chronotype and school start time—it's not just a matter of early birds doing better when school starts early. But, at the same time, the results indicate that there's never a time of day when the students with the latest chronotype outperform the early birds.

But there are at least two ways to look at that finding. One is that the early birds have a general academic advantage and get an extra boost when the school schedule matches their chronotype. While the latter advantage goes away as the chronotype mismatch gets larger, the former stays with them, allowing them to maintain parity at later school start times. Another way focuses on the finding that everyone always has a bit of social jet lag and suggests that morning people simply deal with it a bit better, which offsets the benefits that later chronotypes might see from later school start times.

Why any of this ends up being specific to math is still a mystery.

The difficulty in figuring out exactly what the results are telling us is really a sign that we need replications of this work. Even though this was a large study population by social science standards, dividing it up by three (class start time) and then by two (age group) means that the individual populations being analyzed were still quite small. That could mean that additional clarifying effects are still buried in the statistical noise—or some of the results seen here were spurious.

Despite the considerable uncertainties, though, the result is clearly consistent with past studies that showed that we're simply starting school at a time when it disadvantages a number of students. While the study shows that students can and do adjust their chronotypes in response to the demands of an early start, there are clearly some students who struggle to do so.

Nature Human Behavior, 2020. DOI: 10.1038/s41562-020-0820-2 (About DOIs).