The lat­est think­ing on how nat­u­ral light is vi­tal to health, our daily rhythm and get­ting a good night’s sleep

The Guardian - Review - - Non-fiction - Cas­par Hen­der­son

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

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