How do our bodies tune into the Sun’s rhythm?

It’s not easy to enjoy a good night’s rest: more than one in five peo­ple in France are said to suf­fer from chron­ic sleep dis­or­ders. Yet, “our great-grand­par­ents had much few­er prob­lems sleep­ing,” says Claude Gron­fi­er, a chrono­bi­ol­o­gy researcher at the French Nation­al Insti­tute of Health and Med­ical Research (INSERM) at the Neu­ro­science Research Cen­tre in Lyon. Could it be because they spent their days work­ing out­side? Perhaps.

Since the 1970s and the dis­cov­ery of the first “clock gene”, we have known that our bod­ies are set to the 24-hour day right down to the deep­est lev­el of our DNA. “It is a fine­ly tuned self-reg­u­lat­ing mech­a­nism of mol­e­c­u­lar loops,” explains the neu­ro­bi­ol­o­gist, who is also pres­i­dent of the Société Fran­coph­o­ne de Chrono­bi­olo­gie. A gene codes for a pro­tein, which accu­mu­lates in the cyto­plasm of the cell, before it enters the nucle­us to inhib­it the expres­sion of the orig­i­nal gene until it dis­ap­pears. Then the cycle begins again.

“Since then, we have dis­cov­ered about fif­teen of these ‘clock genes’: TIM, CLOCK, BMAL, REVERB, PER 1, PER 2, PER 3, CRY, etc. Some act as brakes, oth­ers as accel­er­a­tors of the clock,” explains Claude Gron­fi­er. This inter­nal clock is expressed in the suprachi­as­mat­ic nucle­us locat­ed at the base of our brain, fol­low­ing the unchang­ing rhythm of approx­i­mate­ly 24 hours… and 10 min­utes on aver­age in humans. A slight delay cor­rect­ed by our reti­nas! Dai­ly expo­sure to nat­ur­al light syn­chro­nis­es our cir­ca­di­an cycle, adapt­ing it to the alter­na­tion of day and night. With­out light, our entire body is in danger.

The case of the visu­al­ly impaired¹, stud­ied in the 2000s, sheds light on the con­se­quences of a “free-run­ning” clock, i.e. one that is unable to syn­chro­nise itself. “Take the exam­ple of a blind per­son­’s inter­nal clock of 24 hours and 30 min­utes. Their bed­time will be per­fect­ly syn­chro­nised with the real time only every 48 days. In these sit­u­a­tions, gen­er­al prac­ti­tion­ers find them­selves mak­ing long-term pre­scrip­tions for these peo­ple who suf­fer from sleep dis­or­ders, of course, but also from diges­tive prob­lems, drowsi­ness, insom­nia, or anx­i­ety,” explains Claude Gronfier.

A gen­er­alised dis­rup­tion of the body that can be explained by the pres­ence of clock genes well beyond our brain. “They are found in all our tis­sues: lungs, heart, liv­er, mus­cles, adi­pose tis­sue, etc.” Thus, the “inter­nal clock” has now giv­en way to the term “cir­ca­di­an sys­tem” (edi­tor’s note: cir­ca: close to; diem: day), which is bet­ter able to encom­pass all the process­es involved in the wake and sleep cycle. “These periph­er­al sys­tems allow for fine-tun­ing of the cir­ca­di­an rhythm at a local lev­el” the researcher explains. A genet­ic mech­a­nism (between 8 and 20% of the genome) that is thus expressed in rhythm, orches­trat­ed by the cen­tral cir­ca­di­an clock, the only one capa­ble of syn­chro­nis­ing with nat­ur­al light. A neces­si­ty that goes against our cur­rent lifestyles.

“It’s bare­ly 100 years since we first start­ed liv­ing indoors,” recalls Claude Gron­fi­er. Although the chrono­bi­ol­o­gist has tak­en care to posi­tion his office near a large bay win­dow, he points out that this is not enough. “When fac­ing a win­dow, there should be around 300 to 1,000 lux [Edi­tor’s note: lux is the unit of mea­sure­ment for illu­mi­nance]. How­ev­er, our species evolved out­side! We devel­oped in sun­light, which reach­es lev­els of 10,000 to 100,000 lux dur­ing the day.”

Could our sleep dis­or­ders be linked to this lack of light expo­sure? In any case, that is what a 2011 study³ by Damien Léger, head of the Cen­tre du som­meil et de la vig­i­lance de l’Hô­tel-Dieu de Paris, sug­gest­ed about RATP employ­ees. By com­par­ing the qual­i­ty of sleep among bus and tram dri­vers – who dri­ve out­side – and that of their coun­ter­parts con­fined to the metro, the researcher and his team observed a high­er preva­lence of sleep dis­or­ders (insom­nia, day­time sleepi­ness, hyper­som­nia) among the lat­ter. Oth­er stud­ies car­ried out since then con­firm the major role played by nat­ur­al light in our good health. This nat­u­ral­ly rais­es the ques­tion of night work. Beyond shift­ing rest hours, does almost no expo­sure to day­light have a last­ing impact on health?

While the reper­cus­sions of night work⁴ on sleep dura­tion and qual­i­ty are well estab­lished, the lat­est stud­ies also point to poten­tial harm­ful effects on all the major func­tions of our body. “It is not sur­pris­ing, giv­en the vital role of the cir­ca­di­an sys­tem in the body, that these work­ers are at high­er risk of health prob­lems,” says Claude Gron­fi­er. In a study he led with ANSES, the researcher signed a report in 2016⁵ with a group of 19 experts on the health con­se­quences of shift work (edi­tor’s note: work where teams take turns at the same sta­tion at set times). In par­tic­u­lar, it shows a high­er preva­lence of sleep dis­or­ders, depres­sion, anx­i­ety, stroke, obe­si­ty, dia­betes, breast can­cer, an increase in cog­ni­tive dis­or­ders and the occur­rence of car­dio­vas­cu­lar prob­lems. These con­se­quences are cer­tain­ly under­es­ti­mat­ed, as they are not well known by shift work­ers them­selves, who nev­er­the­less rep­re­sent 20% of employ­ees in France.

“One might think that we end up adapt­ing to night work by becom­ing noc­tur­nal ani­mals, but this ignores the fact that with every hol­i­day, every week­end, every social occa­sion, we are once again exposed to sun­light, which resyn­chro­nis­es us to work­ing by day and sleep­ing by night. We are diur­nal ani­mals and not made for night work,” con­cludes the researcher.

Sophie Podevin