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