Everyone who once traveled between countries with different time zones experienced some level of fatigue or confusion — jet lag.

But you don’t need to travel to get jet lag. According to a new study, many teenagers suffer from social jet lag. This especially occurs when there are big differences between the number of hours they sleep at the weekend and in the week, the researchers found.

A study by researchers in Argentina showed that teenagers that go to the school during the morning shift have lower academic performance and up to four hours of social jet lag.

The study, published in the journal Nature Human Behaviour, was analyzing more than 700 students from the Carlos Pellegrini school in Buenos Aires. The sample included 30 courses, 15 from the first year and 15 from the last year, half morning courses and half evening courses.

The researchers were able to link the chronotype of the students (essentially the circadian rhythms), figuring out which were more likely to be active in the morning or in the evening. They also tracked academic performance, measured by the grades obtained by the students.

“We observed that the students with a morning predisposition had a better performance in the morning shift than those who are more nocturn,” Juliana Leone, a researcher at the Torcuato Di Tella University and author of the study, said. “This suggests that academic results are better when the chronotype and the school time match.”

The researchers gave the students a questionnaire and asked about their sleeping times during weekdays, the time they go to bed, the time they get up and how long do they take to sleep. With this information, they were able to build four variables or indicators of sleep.

The variables were the chronotype of the teenagers, the total number of hours of sleep, the social jet lag and the proportion and duration of naps. By connecting the data with the grades, the researchers realized that teenagers going to the morning shift slept very little and had high levels of social jet lag.

“We discovered that teenagers in Argentina have more evening chronotype than in other countries. Nevertheless, we saw differences between shifts,” Leone said. “A general recommendation would be that school should start later, that would be better for all teenagers.”

An intermediate solution suggested by the researchers was that only the older students in the last years of school are the ones to start late, as they were seen to be the most affected. The school could also assign shifts according to the chronotypes of teenagers, they said.

Daniel Pérez-Chada, professor at the Austral University, who didn’t participate in the study, told La Nacion newspaper, that the research has many virtues. He highlighted the size of the sample and that it was done randomly, also agreeing with the idea of starting school later.