Trying to beat the clock

Rhythms of Life: The Biological Clocks that Control the Daily Lives of Every Living Thing

Profile: 2004. 320 pp. £20 1861972350 | ISBN: 1-861-97235-0

“We eat when we are hungry, sleep when we are tired.” These are the first words of a new book that describes one of the most fascinating subjects of modern biology — the biological clock. In the triviality of these first words lies one of the reasons why science has neglected this subject for most of its history: it seems so banal that we get up when the Sun rises and go to bed when it sets.

Although Jean-Jacques de Mairan discovered in 1729 that daily rhythms in plants are independent of the Earth's light–dark cycle, the scientific birth of clock research lies in the middle of the nineteenth century. coinciding with the invention of the light bulb. One could argue that Thomas Edison is the godfather of circadian research, because it was with growing independence from sunlight that scientists became aware of the fact that the daily structure of our behaviour has a life of its own. Circadian research picked up pace throughout the twentieth century, culminating in the discovery of clock genes around the turn of the century.

In their book Rhythms of Life, Russell Foster and Leon Kreitzman succeed in walking a tightrope — although clearly understandable to the lay reader, the detailed description of the biological clock will also be fascinating for scientists. The authors take readers on a captivating journey, walking them through the history of clocks, both mechanical and biological, and guiding them through the spinney of terminology. They lead them on a hunt through tissues and cells, molecules and genes, in a search for the location of the biological clock, and for the parts that make it tick.

When shielded from the outside world, the biological clock runs free at its own pace. Depending on the organism and the individual, the period is slightly shorter or longer than 24 hours (hence circadian, meaning ‘about one day’). The circadian clock is synchronized to the 24-hour daily cycle by environmental signals or zeitgebers (a German term meaning ‘time givers’), predominantly by light.

For most of us, light is the basis for viewing the world, yet there is more to light perception than just vision. This discovery is one of the most fascinating achievements of chronobiology, and Foster is the pioneer who made it possible. Vision relies on local contrast, and requires high spatial and temporal acuity. But the circadian clock, and probably many other biological functions, simply needs to detect whether there is light, and how much. In the course of deciphering the light input to the circadian clock, a completely new light-sensing machinery was discovered. The clocks of mice whose retinae are completely devoid of rods or cones, which are needed for vision, still respond perfectly to light signals. It is a shame that the coverage of this fascinating story is kept relatively short — I hope that this will be the subject of another book.

After explaining the biological basis for daily timing, Foster and Kreitzman bring us back to civilization. With the help of light and the biological clock, our behaviours and all of the functions in our body are placed at the ‘right’ time of day. Humans are day-active, and as such have specialized senses (and a colourful appearance) that we share with other day-active animals — these characteristics are different from those of night-active creatures. But are we really safe from being active at the wrong time of day?

Modern man has created an indoor society, efficiently buffering environmental changes and, at the same time, choosing sustained dimness. Outside light intensities range between 10,000 and 100,000 lux (depending on cloud cover), whereas inside light rarely offers more that 100–400 lux. So the strength of the zeitgeber light, which is so important in synchronizing our biological clock, has decreased enormously as a result of industrialization. Humans have successfully conquered every available biotope, and the independence from sunlight now allows us to conquer every temporal niche of day and night.

We experience the consequences, for example during shift work or jetlag. Over millions of years, our biological clock has learnt to interpret that we experience the brightest light during the day, independent of when we are active. But the biological clocks of shift workers (some 20% of the working population in the West) compare the dim light during the night shift with the bright light of day, and refuse to be fooled. As a result, shift workers and travellers across time zones eat (and work) when they are tired and sleep when they are hungry.

We know that our industrialized lifestyle has many consequences. Living against our biological clock and shielding ourselves from daylight may be among the most important. It is time that the profound knowledge of clock research reaches everyone in our society. This book does an excellent job in this campaign.

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Roenneberg, T. Trying to beat the clock. Nature 427, 784–785 (2004).

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