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107 - Effects of summer time on biological clock greater than expected

Publication in Current Biology
25 October 2007

A quarter of the world’s population is subjected to summer time every year. Thanks to this measure, which was introduced in the twentieth century, we can enjoy daylight longer on summer nights. However, not much was known thus far about the effects of this changing of the clock on the human body. Scientists from the University of Groningen and the Ludwig-Maximilians-Universität Munich have now proven that summer time has prolonged and rather serious effects on our biological clocks. Their results are published in this week’s Current Biology.

The research project was based on an online questionnaire (available on on sleeping behaviour, which was filled in by more than 50,000 people from around the globe. This questionnaire, the Munich ChronoType Questionnaire (MCTQ), was designed to discover the distribution between morning and night people. However, analysis of the collected data unexpectedly showed that people’s sleeping patterns are seriously disrupted as a result of summer time.

Internal cycle

The human body has its own daily rhythm, also known as the biological clock, covering 24-hour periods. This so-called circadian (‘about day’) cycle manages the sleeping pattern, amongst other things. However, as this cycle does not take exactly 24 hours, it has to be adjusted continuously. Light is an important factor in this process. During winter time (normal time) our internal cycle runs more or less parallel to the light / dark cycle, but this is not the case in summer time: our internal cycle does not adjust to this period. This results in sleeping less and possibly a decreased sleeping quality during the summer months.

Night people

Martha Merrow, Professor of Molecular and Genetic Chronobiology in Groningen, says that the results of previous research into the effects of summer time were inconsistent. The current project was set up on such a large scale that more insight has finally been gained into this issue. In addition to the analysis of MCTQ questionnaires, the researchers also conducted a smaller study into the exact effects of summer time on morning people and night people. They discovered that night people in particular experience adjustment problems when the clock is turned forward for summer time. Both groups’ sleeping patterns are quickly restored once the clock is turned back in the autumn.

Seasonal influences

Merrow and her German colleagues also think that the introduction of summer time is one of the factors that have contributed to the decrease of seasonal influence on human behaviour, including reproduction, in recent decades. Our biological clock gets so confused with summer time that it finds it harder to determine which season we are in.


The researchers have not yet found out why summer time results in adjustment problems and why it particularly bothers night people. They will be focusing on these questions next. However, since the effect of summer time on our biological clock is much greater than was thus far expected, the researchers are calling for a re-evaluation and perhaps even the abolition of summer time.

Note for the press:

Prof. Martha Merrow, tel. 050 363 2064 (work). E-mail:

Last modified:15 September 2017 3.26 p.m.
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