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Daylight-saving time

Night flight from Marrakech back home

Munich, 10/23/2012

Daylight-saving time ends on 28 October, and clocks have to be set back to standard time. LMU chronobiologist Till Roenneberg explains how the annual shift back and forth perturbs our internal circadian clock, and suggests how best to adapt to the changes. 

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You once compared advancing the clock in the spring to moving from Frankfurt to Marrakech. Can you explain why?
Till Roenneberg:
Consider what actually happens when time “springs forward” or “falls back”. Only the clocks’ hands are shifted, not time itself. In reality, we all go to work an hour earlier in ‘summer time’; we just don’t notice it because clocks are advanced and show us the same time as before. But from the last weekend in March on, we live in a time zone west of our own, and the clock change artificially flattens the annual change in day-length. The net effect is equivalent to taking a trip to the southwest. Essentially, we are sent overnight to, say, Marrakech – without a return flight until the end of October. The changes effectively set our annual biological rhythms back by as much as four weeks.


You have studied the impact of the time shifts on human physiology, taking individual variation in sleep patterns, so-called chronotypes, into account. Do the early rising “larks” and the night-loving “owls” differ in their reaction to the shifts?
The clock change – notably, not the time change – favours early chronotypes and is detrimental to the late ones. Everyone’s biological clock adjusts to the increase in day-length and the earlier appearance of the sun in spring. Especially the “owls” benefit, because they find getting out of bed easier than during the winter. One has to know that about 80% of the population need an alarm clock to help them rise in time for work from their slumbers – left to themselves they would sleep longer. The clock change in spring cancels the effect of the natural adjustment of the biological clock to longer days, because it basically shifts late chronotypes back into winter mode. Early chronotypes, on the other hand, are happy because they can go to work earlier.


Young people are prototypical owls, being late sleepers in both senses of the term. Is the transition more of a burden for them than for older adults and younger children, who tend to be on their toes at five in the morning?
That young people between the ages of 16 and 22 are the latest chronotypes over the course of a lifetime is a biological rather than a social phenomenon. In early childhood, the body clock has an early set-point, and then advances progressively throughout puberty and adolescence. From then on, the process is reversed until – in old age – we suffer from so-called “senile insomnia”. Being late chronotypes, young people indeed suffer most from the change in clock time, because it is particularly difficult for their body clock to adapt to the new schedule. The biological clock being out of sync with the social clock leads to what we call “social jetlag”, which can result in insomnia, obesity, depression, and in addictions to caffeine, nicotine and alcohol.


You have been arguing for years that the practice of switching back and forth should be abandoned, and Russia actually took this step last year. Do you know of any negative effects arising from the still greater difference in clock time relative to the countries of Central Europe?
Russia indeed decided last year not to change the clocks anymore. Unfortunately, the government did not put an end to “summer time”. On the contrary, it decided that summer time should be observed throughout the year. Not surprisingly - from a chronobiological standpoint - people protested. I recently heard from a Russian colleague that, due to the strong opposition, the decision is to be repealed. The money spent on implementing this costly and ultimately pointless measure would have been better invested in research.


Do you have any advice for late chronotypes and others, who find the switch particularly challenging, that might help them to adapt to the changes?
First of all, I should emphasise that we now get back the hour that was stolen from us in the spring. This weekend, we go back to our ‘normal’ time and thereby reduce our social jetlag. The autumn change should therefore be easy to adapt to. The switch in the spring is a different matter, with potentially serious effects on health. The frequency of heart attacks increases markedly around this time.
One factor that facilitates adaptation to both transitions is natural light. To ease the shift in March, one should get out as much as possible in the morning and stay as little as possible in the evening. To help yourself shift back in October, simply do the opposite.



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