Those crazy rhythms
‘Social jetlag’ is only one of the problems we experience when we – and other creatures, come to that – ignore our biological clock
Circadian Rhythms
A Very Short Introduction Russell G Foster & Leon Kreitzman Oxford University Press 2017 Pd, 143pp, illus, bib, ind, £7.99, ISBN 9780198717683 Birds have them. Bees have them. I suspect that even fleas with doctorates would have them. As Foster and Kreitzman note, “Circadian rhythms are found in nearly every living thing on earth”. They control how sunflowers track the Sun. They influence the migration of butterflies and birds. And they modulate the timing of heart attacks, the accuracy of badminton serves and even how well some anti-cancer drugs, vaccinations and cholesterol-lowering medicines work.
Circadian rhythms – biological changes over the course of a day – allow organisms to synchronise their activities and behaviour with threats and opportunities in the environment, such as food availability, the risk of encountering predators and chances to mate. Cues such as light ‘entrain’ the body to follow the environment.
But almost every cell also has a timekeeper, controlled by a master clock in the brain (the suprachiasmatic nucleus), which is supplied with nerves from the eye. Without cues (such as living underground or during the Artic winter), our biological clocks maintain a circadian rhythm, though it is, on average, 24 hrs 10 mins. The cues fine-tune the pattern to the environment, resulting in, for example, our sleepwake patterns.
We’ve recognised circadian rhythms for millennia. In the 4th century BC, Androsthenes of Thasos, one of Alexander the Great’s admirals, noted that the leaves of the tamarind tree curled and opened over the course of the day. Hippocrates and Galen recognised that fevers often follow a 24-hour pattern. However, scientists have only recently begun to understand the complex biological pathways that set the beat for these ubiquitous rhythms.
Foster and Kreitzman clearly describe our current understanding – and the unanswered questions. All clocks – mechanical or biological – must compensate for changes in temperature. The activity of most biological processes doubles with, within reason, each 10˚ increase in temperature. (You may remember this as the Q10 temperature co-efficient.) Circadian rhythms do not share this temperature dependence, though how they circumvent Q10 is not clear. Such discussions are of more than just academic interest. As Foster and Kreitzman point out, “Being forced to live against one’s circadian clock” can damage your health. ‘Social jet lag’ refers to the difference between the time that alarm makes you crawl out of bed and your natural wake time. Every hour of social jet lag increases the risk of being obese or overweight by 30%.
In addition, true jet lag and sleep deprivation contribute to accidents, illness and poor physical performance. In one study, the more time zones a person who is prone to mental illness crossed, the greater their risk of being admitted to a psychiatric hospital. Even horses and bees suffer jet lag. Bees flown from France to New York went searching for nectar on Paris time – and discovered the flowers in the Big Apple had yet to open. Circadian rhythms also help synchronise seasonal activity. Birds and butterflies migrate. Deciduous trees bloom and shed their leaves. Until recently, the seasons influenced human biology. Researchers have found “indications of annual cycles” in the composition of fat and blood, reproduction, immune activity, disease and death. The activity of about a quarter of our genes changes over the year: some are more active in winter; others in summer. Despite environmental and social changes (electric light, central heating and easy food availability – in the ‘developed’ world at least), the deeply entrenched annual cycles may still influence our biology or the likelihood of developing certain diseases. Circadian Rhythms is an ideal companion to Sleep, another excellent book in the same series and co-authored by Foster. I have a pile of the Short Introductions and they are uniformly excellent: an ideal way to dip your toe in a topic and, being relatively cheap, allow you to decide whether invest in more expensive tomes. They’re well written by leaders in their area, thought-provoking and insightful – and Circadian
Rhythms is no exception. Foster and Kreitzman delve into the molecular biology that underlies the circadian rhythms, but they do so clearly and gradually. So even if you’re not a biologist you’ll be able to follow the discussions. Yet thousands of years after Androsthenes considered the leaves of the tamarind tree and while we’ve made impressive progress, we still have much to learn about this enigmatic biological process.