How to improve sleep, according to science

Doc­tors recom­mend an aver­age of 7 to 9 hours sleep for adults. Yet in France, we struggle to reach the lower end of this range. What can we do to try and improve our night’s sleep? Let’s take a look at some recent sci­entif­ic findings. 

We are sleep­ing less and less. The annu­al sur­vey by the Insti­tut nation­al du som­meil et de la vigil­ance¹ reveals that in 2024, the French will sleep an aver­age of 6 hours 42 minutes dur­ing the week and 7 hours 25 minutes at week­ends, i.e. 15 minutes less a day than in 2023. Although this down­ward trend has accel­er­ated in recent years, it is not a recent phe­nomen­on: we are thought to have lost 1 and a half hours of sleep a day over the last 50 years. How­ever, the same sur­vey shows that we are aware that adopt­ing a healthy life­style helps us to get a bet­ter qual­ity of sleep. And in fact, cor­rect­ing our habits is the first step recom­men­ded by doc­tors and med­ic­al asso­ci­ations in treat­ing sleep dis­orders, before con­sid­er­ing any kind of medication.

To under­stand the impact of a healthy life­style on our nights, it helps to look at the bio­lo­gic­al mech­an­isms that gov­ern sleep. There are two factors that reg­u­late the dur­a­tion, tim­ing, and qual­ity of sleep: sleep pres­sure and the body clock. The first involves so-called homeo­stat­ic pro­cesses, which work like an hour­glass that is turned over when we wake up. Aden­osine plays a cent­ral role here: pro­duced dur­ing the wak­ing phases, this product of ATP break­down (the cells’ main source of energy) accu­mu­lates through­out the day until it reduces neur­on­al excit­ab­il­ity and induces sleep.

Our body also has a “cent­ral clock” loc­ated in the hypo­thal­am­us, dir­ectly linked to sleep struc­tures. Thanks to this clock, the pineal gland (an endo­crine gland in the shape of a pinecone – hence its name – loc­ated at the heart of the brain, which plays a cent­ral role in reg­u­lat­ing bio­lo­gic­al rhythms) secretes melaton­in – also known as the “sleep hor­mone” – at the end of the day, when the light fades, which helps us to fall asleep. Dur­ing the wak­ing phases, the same clock inhib­its melaton­in pro­duc­tion and prompts the adren­al glands to secrete a hor­mone with the oppos­ite effect, cortisol. It stim­u­lates gluc­o­n­eo­gen­es­is to main­tain blood gluc­ose levels and sup­ply energy to the cells. Like a con­duct­or of an orches­tra, our intern­al clock also reg­u­lates body tem­per­at­ure and syn­chron­ises mul­tiple peri­pher­al clocks gov­ern­ing vari­ous bio­lo­gic­al func­tions such as appet­ite rhythms or intest­in­al con­trac­tions. The cent­ral clock is itself con­stantly reset to the day/night cycle by extern­al syn­chron­isers: mainly light, but also (with more mod­est influ­ences) phys­ic­al activ­ity, meal­times, social inter­ac­tions, and out­side temperature.

This is how our body clock defines the win­dows of sleep when sleep qual­ity is best. To rein­force this, it is essen­tial to main­tain reg­u­lar bed­times and wake-up times and to listen for the first signs of sleep­i­ness. But oth­er strategies can also be put in place.

Doc­tors recom­mend expos­ing one­self to “blue sky” first thing in the morn­ing, in order to stim­u­late cortisol pro­duc­tion and inhib­it melaton­in pro­duc­tion. Syn­chron­isa­tion with light involves activ­at­ing the photore­cept­ors in our ret­ina, the melan­op­sin cells, which are more spe­cific­ally sens­it­ive to blue light. Con­tem­plat­ing the clouds through your office or flat win­dow won’t be enough. This is because light levels rarely exceed a few hun­dred lux, where­as the stand­ard recom­mend­a­tions are 10,000 lux (equi­val­ent to the illu­min­a­tion of a shaded area in sum­mer) for at least half an hour. On the oth­er hand, and again to help our clocks work in sync, it’s very import­ant to dim the lights and avoid look­ing at screens in the even­ing (stud­ies sug­gest 30 minutes to 2 hours before bed­time), as they emit light that is rich in blue.

A recent study con­duc­ted by Inserm, the res­ults of which have been pub­lished in the Journ­al of Pineal Research, refined these recom­mend­a­tions². Using two sep­ar­ate groups of volun­teers, with an aver­age age of 25 and 59, the study looked at melaton­in pro­duc­tion fol­low­ing expos­ure to dif­fer­ent col­ours. It appears that while melan­op­sin is the only photore­cept­or involved in inhib­it­ing pro­duc­tion of the hor­mone in young sub­jects, in older sub­jects oth­er photore­cept­ors, sens­it­ive to dif­fer­ent col­ours, also come into play. This mech­an­ism is thought to be an adapt­a­tion to the brown­ing of the crys­tal­line lens that occurs with age, which reduces the amount of light reach­ing the ret­ina. These res­ults there­fore sug­gest that the eld­erly would par­tic­u­larly bene­fit from daily expos­ure to day­light, which is rich­er in wavelengths than arti­fi­cial light. The research­ers also recom­mend adapt­ing the col­our and bright­ness of indoor light­ing if going out is impractical.

The sci­entif­ic lit­er­at­ure is full of con­sist­ent evid­ence of a pos­it­ive cor­rel­a­tion between phys­ic­al activ­ity and bet­ter sleep. This cor­pus includes two stud­ies pub­lished in 2024, one by the Uni­ver­sity of Reyk­javik³, the oth­er by the Uni­ver­sity of South Aus­tralia⁴. In the Iceland­ic study, 4,339 par­ti­cipants from 9 European coun­tries aged between 39 and 67 were mon­itored over a 10-year peri­od. Levels of phys­ic­al activ­ity, dur­a­tion of sleep and sleep dis­orders were assessed using ques­tion­naires. Con­clu­sion: people who main­tained reg­u­lar phys­ic­al activ­ity over the study peri­od repor­ted few­er prob­lems fall­ing asleep or extreme sleep dur­a­tions (less than 6 hours or more than 9 hours) than the most sedent­ary sub­jects. The Aus­trali­an study, based on an assess­ment of the activ­ity para­met­ers of a pop­u­la­tion of 1,168 chil­dren and 1,360 adults, over 8 days and 24/24, shows that mod­er­ate to vig­or­ous phys­ic­al activ­ity dur­ing the day is cor­rel­ated with longer, bet­ter qual­ity sleep. The research­ers believe that this effect is due to an increased secre­tion of sero­ton­in, which helps to improve both mood and the pro­duc­tion of melatonin.

Oth­er stud­ies also sug­gest that, where pos­sible, reg­u­lar prac­tice times should be observed, avoid­ing those before bed­time. A study con­duc­ted by the Uni­ver­sity of Caen-Nor­man­die and pub­lished in 2024⁵ may qual­i­fy this recom­mend­a­tion. Car­ried out on 16 young adults in good health, it sug­gests that 30 minutes of mod­er­ate phys­ic­al activ­ity, one hour before bed­time, has, in this pop­u­la­tion at least, only a very min­im­al impact on sleep effi­ciency. But more research is needed to con­firm these results.

It’s gen­er­ally accep­ted that it’s a good idea to avoid drinks con­tain­ing caf­feine in the even­ing. Why? Because caf­feine binds to the same recept­ors as aden­osine, delay­ing the feel­ing of tired­ness. As for the con­tents of your plate, a study con­duc­ted by the team led by Armelle Ran­cil­lac, a research­er at Inserm and the Collège de France, showed that a gluc­ose-rich din­ner favoured sleep⁶. The research­er is inter­ested in gli­al cells, which are more numer­ous than neur­ons and essen­tial to their func­tion­ing. Her work focuses more spe­cific­ally on astro­cytes, so named because of their star shape. His research has shown that these cells, which cap­ture blood gluc­ose, can induce a release of aden­osine that var­ies accord­ing to the time of day. Astro­cytes take account of the time of day to adapt their response to the same increase in gluc­ose. This response is great­er in the even­ing than in the morn­ing, to encour­age sleep at the end of the day.

Anne Orliac