Last Friday, I was in Warsaw with a large group of researchers to bring together different findings on the global decline in learning outcomes. Many different explanations were discussed. I added a new one myself, which I hope to tell you more about soon. In the meantime, I would like to share the already published research behind one of Friday’s more striking presentations, by the Danish researcher Christian Christrup Kjeldsen. And that research is entirely about how tired our 10-year-olds are! Yes, another post about the importance of sleep.

Kjeldsen and his colleagues used the enormous amount of data from PIRLS and TIMSS. They examined the results for 10-year-olds (fourth graders) from 39 education systems that participated in PIRLS in 2016 and 2021. They also included data from TIMSS 2019 and 2023. Altogether, this amounts to more than 9 million student observations. Each time, the children in these international assessments got the same simple question: how often do you feel tired when you arrive at school? They could answer every day, almost every day, sometimes, or never.

And the trend is clear. In 2016, 15.8 per cent of the children said they arrived at school tired every day. By 2023, that had risen to 25.2 per cent. At the same time, the proportion of children who said they were never tired fell from 17.8 per cent to 12 per cent. So, in 2016, about one in six children were tired every day. Seven years later, it was about one in four. Am I allowed to think that is a lot?

Boys appear to be more tired than girls. In 2023, 27 per cent of boys said they were tired every day, compared with 23.5 per cent of girls. More importantly, however, the increase is clearly visible in both groups.

And then there is the link with learning outcomes. Children who say they are tired every day consistently score lower in reading, mathematics and science. The effect sizes range from d = .20 to d = .32. The difference remains even when the researchers take age, gender and socioeconomic background into account.

In their paper, the researchers translate this into several months of learning. But Christian was already a bit more careful about this in his presentation, and rightly so, I think. It was the recurring theme, and also a frustration, throughout the conference: data like these show correlations, not causal relationships. This study does not show that being tired causes lower achievement. It does show that the two are clearly related. But tiredness could just as well be partly a symptom of something else. A child can be tired for many different reasons, and some of those reasons may of course also be related to school achievement. To give just one concrete example, it made me think of something I shared this weekend: whether we, as humans, can actually cope with these busy, complex times.

There is another caveat. The researchers refer to a “sleep crisis” in their title, but, strictly speaking, they did not measure sleep. They don’t know how many hours these children sleep, when they go to bed or how well they sleep. They only know how often children say they are tired upon arriving at school.

So we don’t know why children have become more tired either. Anyone who immediately wants to point to smartphones, social media, pressure at school or COVID is going beyond what these data can tell us. These possible explanations came up quite often on Friday too, but always with the necessary caveats.

What this study does show is a striking development. We can only guess why our 10-year-olds have become more tired. But that so many more children apparently start their school day already tired seems to me something we shouldn’t simply ignore. And, by the way, I’ve heard the same thing said about preschoolers too…