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