Getting teenage school children to bed at a decent time can be a challenge for parents, not to mention waking them up in the mornings.
In the past, the difficulty in getting "lazy" teenage children awake and off to school was attributed by parents and sleep experts to the 'lack of an early night'.
However in recent years, teenage sleep patterns have been found to have a real biological cause. Research in America shows that teenagers have a different internal body clock than fully grown adults and naturally fall asleep later.
They are far more alert late into the night when they are not tired, while they need a lie-in the next day. At weekends they can of course stay up late and have a long lie-in on Saturday and Sunday.
However, when it is time to get up for school on Monday, the feeling can be akin to 'jet lag', with the adverse knock-on effects on performance in the classroom.
A simple solution would be to start classes later in the morning. One school in Malmö has begun experimenting with a 10am start for its teenage pupils every Monday.
"It's about teenagers' biological clock and hormones, and that research says that they need to sleep longer in the mornings. So we decided to try starting at 10am on Mondays and 8am the rest of the week," Oxievång school principal Jenny Nyberg tells Radio Sweden.
"We will evaluate this after this semester. We think it is very good for the students to get extra sleep after the weekend," she says.