'Chasing the Sun' book cover Image Credit: Supplied

Chasing the Sun: The New Science of Sunlight and How it Shapes Our Bodies and Minds

By Linda Geddes, Wellcome Collection, 256 pages, £14.99

Humans have long understood that sunlight can be good for us. Almost 4,000 years ago, the Babylonian king Hammurabi advised priests to use sunlight in the treatment of illness. In the 4th century BCE, Greek physicians associated with Hippocrates recommended it for the restoration of health. The splendid 19th-century geographer and anarchist Elisee Recluse advocated nudity on the grounds that it was healthier for skin to be fully exposed to light and air. What, then, is the “new science” of sunlight?

Michael Pollan sums up his manifesto In Defence of Food with: “Eat food, not too much, mostly plants.” Similarly compressed, Chasing the Sun by Linda Geddes would become something like: “Get lots of natural light, not too much direct sun, and avoid blue light at bedtime.” But that would understate a key point. Because the new science — which Geddes explores in a range of encounters, from insomniacs untethered from the passage of day and night to Nobel-winning scientists and Sami reindeer herders, as well as in her own try-it-at-home experiments — situates an understanding of the importance of natural light in the study of the daily and seasonal rhythms of our lives. We are, as Ted Hughes wrote, “creatures of light”, but for our wellbeing, the dark is just as important.

In Geddes’s vision, respect for the circadian cycle — daily physiological changes that are part of our evolutionary heritage dating back billions of years — is as important as sleep is to Matthew Walker in his bestselling Why We Sleep. “The circadian clock is far more than a biological curiosity,” she writes. In the last two decades, “it has been implicated in pretty much every biological process looked at. There is strong daily rhythm in body temperature, blood pressure and the hormone cortisol ... Circadian rhythms govern the release of chemicals that regulate mood; the activity of immune cells that fight off disease, and our body’s response to food.” A reduction of the amplitude of these rhythms, or their disruption, she argues, is associated with poorer sleep and illnesses from depression to dementia to cancer to cardiovascular disease.

Chasing the Sun is published with the support of the Wellcome Trust, the UK’s largest non-governmental funder of scientific research, which aims “to improve health for everyone by helping great ideas to thrive”. It was written in the wake of the discoveries of molecular mechanisms controlling the circadian rhythm that won researchers the 2017 Nobel prize in physiology or medicine, but other remarkable findings fill its pages.

You may have learned in school that there are two types of light-sensitive cell in the human eye: rods, which provide black and white vision in low-light conditions, and cones, which work in bright light and enable us to see colour. But in 2002 a third type was discovered. Intrinsically photosensitive retinal ganglion cells, or ipRGCs, play no part in vision but act as a trigger for the “master clock” that governs circadian rhythms in the human body, and, undamaged, they work even in the eyes of people who are otherwise completely blind, enabling them to follow a normal wake/sleep cycle.

Florence Nightingale recognised in the mid 19th century that the sick benefited from fresh air and sunlight. Hospital design has, however, largely continued to be poor in this respect. Recent research may bring about change. A study at Loughborough University found that for every increase in daylight of 100 lux (a lux is a standard measure of illuminance, the perceived brightness of light hitting a surface), a patient’s stay in an intensive care unit was reduced by more than seven hours. A study of Canadian patients recovering from heart attacks found that the mortality rate among those recuperating in brighter rooms was 7 per cent, compared with 12 per cent among those assigned gloomier rooms.

The book is most engaging when Geddes writes about her own experience. It starts in Las Vegas, where she is unable to escape the constant stimulation of artificial light designed to induce a wakeful stupor that favours compulsive gambling. Seeking a culture that is comparatively sane, she visits the Amish, who shun electric light (although not necessarily all electrical equipment). With only oil lamps in use, light levels in Amish homes are at least three to five times lower than those in electrified homes — enough for sociability but not enough to delay sleep. Crucially, daylight exposure is much higher among Amish, who mostly work outdoors, than it is for most of us in western countries.

Geddes tries bringing the lesson home. With the help of some friendly researchers, she experiments on herself and her long-suffering family, trying to mimic the Amish lifestyle or go wild camping “except doing it while trying to juggle an office job and a busy family life in central Bristol”. It sounds implausible, but there are marked results. Spending more time outdoors even in winter, she finds her attitude begins to change. The illuminance in a typical office is between 100 and 300 lux during the daytime, whereas even on the gloomiest, most overcast winter’s day it is at least 10 times brighter outside. Geddes finds that for every 100 lux increase in her average daylight exposure, her sleep efficiency (the ratio of time sleeping to time in bed) increases by almost 1 per cent, and she gets about 10 minutes more rest. And there are further benefits. “I registered the beauty of hoar frost on rosehips, and the tranquillity of an empty park on a bright December morning.”

At Dowth, one of the great neolithic tombs in Ireland, sunlight on the winter solstice briefly shines gold on to sun-like spirals and other marks carved on the stones within. Geddes’s reflection on this, as well as on a dark-sky night near Stonehenge, where the stars reveal themselves in glory, helps bring a sense of wonder to this fact-filled book. But there is much to discover, appreciate and concern us that is not covered in Chasing the Sun. One of the most urgent questions — central to wellbeing in its broadest and deepest sense — is whether humanity’s hunger for light and energy can be met without endangering ourselves and the many forms of light-filled beings on whom we depend, and with whom we share the planet.

–Guardian News & Media Ltd