Theresa Lemke is collecting data from a study in which schools have tried adapting to teenagers' "internal body clock" - by starting morning lessons a little later.
Text: Catarina Thepper, first published in Medicinsk Vetenskap nr 4 2025
What is the problem with starting the school day early?
"Between the ages of 13 and 18, there is a biological shift in our sleep pattern. Teenagers therefore feel tired later in the evening and wake up later in the morning, which makes it harder for them to get the 8-10 hours of sleep they need if they have to wake up early. The change is not permanent. The earlier sleep pattern returns as a young adult," says Theresa Lemke , researcher at the National Centre for Suicide Research and Prevention (NASP), Karolinska Institutet.
How are schoolchildren affected by lack of sleep?
"I have been particularly interested in studying how sleep affects mental health. In a previous study we conducted among adolescents in Stockholm, it turned out that students who sleep less are at a greater risk of developing symptoms of depression. They are also at a higher risk of experiencing suicidal thoughts.
"Insufficient sleep affects emotional regulation, which in turn could influence the classroom environment and the risk of conflicts in school. Sleep is also crucial for concentration and learning. These are aspects we want to examine in the upcoming study."
When should school start, ideally?
"Based on what we know today, I would say it is a good idea for lower secondary students to start the school day at nine o'clock. At the same time, you have to be a little cautious, as the research comes from countries where the school day is organised differently, with school buses in the United States, for example. That is why we are now running a pilot study in a few Swedish schools and evaluating the effects."
What do schools and students say about starting the school day later?
"We have carried out interviews with both students and headteachers at Swedish schools. The headteachers agree that lack of sleep among young people is a problem, but believe that screen time, social media and parental responsibility are the causes, rather than the natural changes in teenagers' sleep rhythms. Teenagers generally view a later start of the school day positively, but some worry that it might interfere with leisure activities."
What could the effects of a later school start be?
"A common question is whether a later start simply means that young people go to bed later, but previous research has shown that there is, in fact, a benefit. Starting school an hour later does not necessarily mean an extra hour of sleep, but on average, it means half an hour more. That may sound little, but when it comes to, for example, the risk of depression, we know that half an hour's sleep reduces the risk by ten per cent."
What would be required for later school start times to be introduced in Sweden, in your view?
"Headteachers, who are the ones deciding on later school starts, often state that they want to work in an evidence-based way with student health, but need scientific support to show that a later start really makes a difference. So, I believe the studies we are conducting right now could have an impact."
