Researchers shed light on our internal clocks
Our body's internal daily clocks are closely linked to the local sunlight hours, even though we live and work according to a common 'social time' that is determined by time zones, researchers have found.
The work, which is partly funded through the EU-funded EUCLOCK project, is published in the latest edition of the journal Current Biology.
Our body has an internal clock which structures many aspects of our lives, including rest, activity, vigilance, blood pressure and enzyme activity. The importance of this clock is underlined by a growing body of evidence showing that people whose clocks are disturbed, such as shift workers, have an increased risk of accidents, sleep problems and other health problems.
However, this internal clock needs to be synchronised to the external day, and to do this our body takes cues, known as 'zeitgebers', from the environment. This process of synchronisation is called 'entrainment'.
One of the most important zeitgebers is the sun. However, people now live their lives according to a common time zone, which may be quite different from the local time as calculated by the position of the sun. For example, in Santiago de Compostela, in the far west of the Central European Time (CET) Zone, midnight occurs over an hour and a half before 'mid-dark'.
The researchers wondered whether humans' internal clocks were still synchronised to the true sun time of their home town or the socially imposed clock time as determined by their country's time zone. They studied questionnaires completed by over 20,000 Germans which looked at sleeping and activity patterns on both work and rest days. They then matched this data with the respondent's location, and analysed how this varied across the country.
They found that the internal clocks of people living in sparsely populated areas were linked closely to the sun time, while the effect was weaker for people living in big cities. They attribute this last finding to the fact that urban dwellers are typically exposed to less natural light than people living in more rural areas. They also propose that the social cues in cities may be relatively strong compared to those in rural areas, where the environmental cues prevail.
'Our findings emphasise the importance of individual, circadian time rather than social, external time in scientific studies, in school and work schedules or in medical considerations,' the authors conclude. 'They also demand careful re-examination of how changing to and from daylight saving time affects individuals.'
Most people believe that their own internal clocks can easily adapt to social time cues, and furthermore some studies show that people really do adapt readily to Daylight Saving Time (DST) changes. However, lead author and EUCLOCK Project Coordinator Professor Till Roeneberg of the Ludwig-Maximilians-University in Munich is not so sure.
'If people are obviously sensitive to 36 minutes of dawn/dusk changes as they appear between West and East Germany, a change of a whole hour that only takes place on our clocks but not at the level of dawn and dusk should not readily lead to an adjustment,' he commented. 'We are currently running a large field study investigating the responses of individuals to DST changes. The first round (October change) is currently being analysed and a second round (March change) will follow soon.'
Elsewhere in the EUCLOCK project, researchers are looking at the physiological process behind our bodies' circadian rhythm and how it responds to zeitgebers. They will also study how shift work affects our internal clocks, and it is hoped that the knowledge generated will help shift workers cope more easily. Ultimately the project partners hope that their work will help people to optimise their daily structures and thereby increase health and quality of life.
The issues addressed in the project could have implications for policy makers. During the 20th Century, many European countries switched time zones; for example, Portugal used CET from 1966-76 and 1992-96, but is now on Greenwich Mean Time (GMT).
The UK has experimented with CET, but an increase in accidents in the dark winter mornings meant that the country always ended up going back to GMT. Now Tim Yeo, a Member of Parliament, is introducing a bill to move the country to CET for a trial period. It will undergo its second reading in Parliament on 26 January.
'According to the Royal Society for the Prevention of Accidents, moving the clocks forward in this way would save over one hundred lives every year by cutting the number of road accidents,' said Mr Yeo. 'Recent research from Cambridge University suggests it would also save energy by reducing demand for electricity, and thereby address the threat of climate change by cutting carbon emissions.'
Supporters also cite the fact that more daylight in the evenings will mean more opportunities for outdoor activities such as sport, while companies would be able to work with colleagues and partners in the rest of Europe throughout the day.
Data Source Provider: EUCLOCK Project / Current Biology
Document Reference: Roenneberg, T. et al. (2007) The human circadian clock entrains to sun time. Current Biology. 17: R44-R45.
Subject Index: Coordination, Cooperation; Medicine, Health; Safety; Scientific Research