Our bodies fail to adjust to seasonal clock changes

The UK and other countries have been altering their clocks with the seasons since at least the beginning of the last century.

The argument in favour of Daylight Saving Time (DST), or British Summer Time as it's called in the UK, is clear: daylight hours are effectively shifted from the early morning to the evening, so that there is more time for sport and leisure in the summer months.

Increasingly, however, there have been calls for Britain to remain on British Summer Time throughout the year - to stay coordinated with Europe, and to reduce energy consumption in the dark winter months. Now Thomas Kantermann and colleagues have added to the debate with a study showing that the ability of our bodies to acclimatise to the natural changes in sunrise onset is lost following the artificial shift of the clocks forward each Spring.

Kantermann's team took advantage of self-report data from more than 55,000 people gathered as part of the Munich Chronotype Questionnaire. This showed that the timing of people's sleeping and waking tracks the natural changes to dawn time that occur during the Winter months (i.e. during standard time) but fails to show the same adjustment after the introduction of Daylight Saving Time in the Spring.

In another study, the researchers used actimeters (gadgets strapped to the wrist that measure movement) to track the sleep patterns of 55 volunteers for four weeks prior to, and four weeks after, the clock changes in the Autumn and Spring of 2006-2007. Again, this confirmed that after the advancement of clocks in the Spring, people's sleep patterns failed to adjust to the seasonal shifting of sunrise, as they had been doing before the clock change. This was especially the case for 'owls' - people with a preference for the evening who find it difficult to get up in the morning.

These results reflect the way people suffer more from jet leg when travelling westwards and time is advanced, compared with travelling eastwards, 'backwards' in time.

Co-author Till Roenneberg said: "It is much too early to say whether DST has a serious long-term impact on health, but our results indicate that we should consider this seriously and do a lot more research on the phenomenon."
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Kantermann, T., Juda, M., Merrow, M. & Roenneberg, T. (2007). The human circadian clock's seasonal adjustment is disrupted by daylight saving time. Current Biology, 17, 1996-2000.

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