Spring forward, Fall back
(December 3rd 2007) Hopping back and forth between many time zones across the planet for holidays or business, tens of thousands of people have become accustomed to dealing with jet lags of several hours. It's not a big deal. That makes us wonder why some start whining when the clocks are altered by a single hour for Daylight Saving Time. Melanie Estrella has a look at adjusting the daily routine to new rhythms.
While hardly anyone is able to remember which way round the clocks had to be reset some weeks ago, many people seem to feel that it's a real drag adjusting their daily routine to the new rhythm. People bellyached in newspapers, radio features and TV shows about the confounding of their biorhythms. Is this temporal intervention simply triggering a subjective feeling of disorientation by time-sensitive sissys or does it actually have an impact on our life?
If mankind's fiddling with the natural daylight hours twice annually for Daylight Saving Time (DST) influences our physiology and behaviour, the effects have been the subject of only a few studies so far. This is surprising considering that a quarter of the world's population, including all Europeans other than Icelanders, are affected by these time changes.
To learn more about the effects of this adjustment on our lifestyle, Till Roenneberg, Professor of Ludwig-Maximilian-University in Munich, Germany, and his colleagues examined sleeping and activity patterns in a recent comprehensive survey (Kantermann, T et al. The human circadian clock's seasonal adjustment is disrupted by daylight saving time. Current Biology, Epub 24 October 2007).
Analysing seasonal variations in the sleeping schedule of 55,000 central Europeans, the Bavarian researchers found out that the timing of sleep followed the seasonal progression of dawn under standard time, but not under DST.
Ronneberg's team took a closer look at the sleeping and activity patterns of 50 people for eight weeks around the two DST transitions in a second study. The observed persons were assigned to groups according to their chronotypes: Early risers were categorized as 'morning larks' while people going to bed late, tending to be late risers as well, were marked as 'night owls'.
As it turned out, sleep and activity times as measured by sleep logs and wrist actimeters, respectively, adjusted easily at the end of DST in autumn. However, at the introduction of DST in spring, when clocks are set an hour ahead, the timing of peak activity levels did not adjust. The night owls were particularly bothered by the change. Their biological timing remained on standard time while all of their social activities shifted forward one hour. For the morning larks, this effect was less pronounced.
The reason for the observed phenomenon caused by varying only one of 24 hours of the day is the operating mode of the human internal clock: The body's timepiece requires daylight for synchronisation with the environment. Especially important for setting the inner clock regulating the chronology of physiological and behavioural processes is dawn. While dawn times change rapidly during the spring DST transition, which often occurs close to the March equinox, dawn times change to a lesser extent around the autumn transition, taking place over a month after the September equinox.
So the modern human is not living in his or her natural rhythm - especially during DST and even on its days off. We had already expected something of this sort. Just as we had suspected before it's the early bird that catches the worm and not the owl, howling from dawn till dusk about not quite being up to it, especially at DST transitions.
Whether or not DST-induced alterations of our circadian rhythm has negative consequence on our health is not yet clear. Presumably it will eventually turn out that the natural unaltered dawn is yet another item on the list of things that benefit us most just as they are.
