Body clock and daylight saving time
“The return to GMT this Sunday may help us get back in tune with the natural rhythm of night and day,” reported The Guardian today. The newspaper reports that the switch to daylight saving in the summer time may have “adverse effects” as it “interrupts people’s natural sleep cycle”.
The story is based on analysis of sleep data and a smaller study where people were monitored before and after the clocks went backward or forward in the autumn and spring. During standard time (winter time), sleep patterns tend to follow a rhythm dictated by the dawn, while in summer time (daylight saving time), the timing of sleep doesn’t follow this natural pattern. The study is interesting, but as the implications for health were not considered, there are no conclusions to be drawn on health matters.
Where did the story come from?
Dr Thomas Kantermann and colleagues from the Ludwig-Maximilian-University and the University of Groningen in the Netherlands conducted this research. The study was supported by the following research networks: EUCLOCK and CLOCKWORK. It was published in the peer-reviewed medical journal Current Biology .
What kind of scientific study was this?
This observational study had two parts. In the first part, the researchers undertook a cross-sectional description of people's sleep patterns from data that had been collected from over 55,000 people in Central Europe.
In the second part of the study, 50 volunteers were recruited from Germany, Italy, Switzerland, France, Slovakia, Holland and Luxembourg. They completed a questionnaire which determined their 'chronotype' (the internal factors that determine whether a person is a 'morning' person or an 'evening' person). Sleep logs were used to determine factors including bedtime, time spent preparing for sleep, how alert the person felt at bedtime, time of waking up, time of getting up, using an alarm clock, subjective sleep quality, and whether it was a work day or a free day.
The participants supplied this information for four weeks before and four weeks after both the autumn time change in 2006 and the spring time change in 2007. The researchers then compared the effects of the time change on sleep patterns across the different types of chronotypes.
What were the results of the study?
In the large database of more than 50,000 people, the researchers found that the sleep pattern on free days (i.e. when someone doesn’t have to go to work) follows the natural progression of dawn during winter time (standard time), but not during the summer, when daylight saving time is imposed. These findings were confirmed by the smaller study in 50 volunteers.
In these people, the timing of sleep and activity adjusted well to the return to standard time (in the winter), but not so well to the change to daylight saving time in the summer months. This problem adjusting was particularly pronounced in people who were 'late' chronotypes, i.e. the 'owls' or 'evening' people, compared with those who had the 'early' chronotype – the 'larks'.
What interpretations did the researchers draw from these results?
The researchers say their study shows that the human body clock doesn’t adjust to daylight saving time and that this interrupts the ability of the body to adjust to the change in the length of daylight during the seasons.
What does the NHS Knowledge Service make of this study?
This is an interesting study showing that our body clock doesn’t adjust absolutely to the change in the clocks that happens in spring and introduces daylight saving time. This study did not look at the effects of this failure to adjust on any health matters. We need further research before we can assess the impact of these disruptions in circadian rhythms on health.
The researchers have collected a lot of data, but it is difficult to see how this leads to any advice now on how we might avoid any difficulty with waking up in the dark.
Sir Muir Gray adds...
This is very low on my list of worries; I won’t lose any sleep over this finding.
