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Prof. Martha Merrow: 'Teenagers victims of their biological clock'

14 October 2008

Adolescents are lazy, stay out far too late and then can’t be got out of bed in the mornings. Just teenage recalcitrance? Not according to Martha Merrow, an expert on the biological clock and the resulting behaviour. ‘Adolescents are usually evening people in the extreme as a result of the developments their biological clocks are going through. Schools should learn to take this into account. For example by letting the school day start later.’

The phenomenon of the biological clock is nothing new. Every human body has a ‘built-in’ daily rhythm. Our biological clock tells us or directs us when it's time to sleep, for example. But the biological clock is not set at the same time for everyone. That has a number of causes. Your genes, for example, play an important role. They determine whether you are an evening person, a morning person or something in between. Another important aspect is the amount of light that you encounter. ‘What many people don’t realise, however, is that age also plays an important role,’ says Merrow.

Night owls

In puberty, adolescents are going through many changes. Merrow: ‘Research we’ve conducted with about 80,000 people of all ages shows clearly that the chronotype shifts further and further. It gets later and later, until people reach the age of about twenty. That peak is wonderful to watch. Suddenly adolescents change into true night owls. Even if they’d never been one before. There seems to be no escaping it. If you just look at their biological clocks, it's really strange that all those schoolchildren are expected to get up at seven o’clock to get to school on time.

Not lazy

‘Teenagers are not lazy, they are victims of their biological clocks,’ states Merrow. ‘That is a prejudice that many night people have to contend with. You can barely escape the influence of your biological clock. Going to bed before you really feel like it never works. I regularly hear from people that they then lie staring at the ceiling for hours. Evening people who have to adjust to the usual rhythm of getting up early are thus simply not getting enough sleep. And that immediately affects your concentration. In addition, the switch between sleeping less during the week and longer on free days causes a condition that Merrow calls ‘social jetlag’.

Natural daily rhythm

Merrow herself never starts her lectures before ten in the morning. ‘I just know that my students are not able to concentrate properly before that time. The same applies to pupils at secondary school. If you want profit from optimum concentration and performance from the pupils, it would be better to start the school day later. That would fit much better with the natural daily rhythm of most of the adolescents.’

Not a trivial decision

Of course, you can’t change the starting time of a school day from one day to the next. Merrow: ‘I do realise that this is certainly not a trivial decision. It may sound simple, but there’s a huge adjustment to be made. You just have to look at parents who take their child to school and then go on to work. They would have to adjust their working hours too. But it is something important that should be taken into consideration during the various educational reforms.’

Curriculum Vitae

Martha Merrow (1957) studied Biology at Middlebury College and did her PhD research on immunology at Tufts University Medical School (PhD awarded in 1991). She then became a researcher at Dartmouth Medical School (USA) and the Ludwig-Maximilians-Universität in Munich, where she began to specialize in chronobiology. She came to Groningen on a Rosalind Franklin Fellowship and has been professor of Molecular and Genetic Chronobiology at the Faculty of Mathematics and Natural Sciences of the University of Groningen since 2006. For more information about Merrow’s research, go to You can find out what chronotype you are by completing the Munich Chronotype Questionnaire (MCTQ at


Prof. M. Merrow

Last modified:15 September 2017 3.11 p.m.
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