The latest thinking on how natural light is vital to health, our daily rhythm and getting a good night’s sleep
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 19thcentury geographer and anarchist Élisée 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 Sámi 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. by Linda Geddes, Profile, £14.99
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%, compared with 12% among those assigned gloomier rooms.
Chasing the Sun: The New Science of Sunlight and How It Shapes Our Bodies and Minds