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One in three French adults is thought to have a sleep disorder. While the prevalence of these disorders increases with age, the biological mechanisms at play are relatively unknown, leaving scientists in doubt as to their origin. In a new study, Inserm researcher Claude Gronfier and his team at the Lyon Neuroscience Research Center (Inserm/CNRS/Université Claude-Bernard Lyon 1) hypothesised that their onset during ageing was linked to a desynchronisation of the biological clock caused by decreased light perception. In the course of their research, they identified a new adaptive mechanism of the retina during ageing that enables older individuals[1] to remain sensitive to light. These findings are also of clinical relevance in encouraging older people to have more exposure to daylight, rather than artificial light, to avoid developing sleep disorders. These results have been published in the Journal of Pineal Research.

Almost all biological functions are subject to the circadian rhythm, which is a 24-hour cycle. The secretion of the night hormone melatonin is typically circadian. Its production increases at the end of the day shortly before bedtime, helping us to fall asleep, and falls before we wake up.

Previous studies have shown that its secretion by the brain is blocked by light, to which it is very sensitive. This sensitivity to light can manifest as desynchronisation of the circadian clock, which can lead to sleep disorders. Other studies have also revealed the important role, in the control of melatonin production, of melanopsin – a photoreceptor present in certain cells of the retina which, being highly sensitive to light (mainly blue light), regulates pupillary reflex and circadian rhythm. Therefore, when exposed to light, melanopsin becomes a driver of melatonin suppression and biological clock synchronisation.

While sleep disorders are already common in adults, they increase with age: nearly one third of people over 65 chronically consume sleeping pills[2]. Yet there are no previous studies specifically focusing on the biological mechanism at work in age-related sleep disorders. Are we talking about the consequence of a problem of light perception? If so, at what level? And what is the role of melanopsin in this specific case?

A team at the Lyon Neuroscience Research Center tried to elucidate this mystery. The scientists observed the effects of light on melatonin secretion in a group of adults. The participants were all exposed to 9 different coloured lights (corresponding to 9 very precise wavelengths) to enable the scientists to identify the mechanisms involved via the photoreceptors concerned.

The participants were divided into two distinct groups, with mean ages of 25 and 59. This experiment was performed in the middle of the night, when the body normally releases the most melatonin.

The results show that, out of the lights tested, blue light (with a wavelength of approximately 480 nm) is very effective in suppressing melatonin production in the youngest individuals. More specifically, the scientists observed that in the young subjects exposed to blue light, melanopsin was the only photoreceptor driving melatonin suppression. Conversely, in the older participants, photoreceptors other than melanopsin appear to be involved, such as the S and M cones – photoreceptors that enable the world to be perceived in colour, and which are located in the outer retina.

These data suggest that while ageing is accompanied by decreased melanopsin involvement in visual perception, the retina is able to compensate for this loss through an increase in the sensitivity of other photoreceptors that were previously not known to be involved in melatonin suppression.

These observations enable the scientists to conclude that light perception – and light requirements – change with age.

While for young people, in whom only the melanopsin receptor is involved, exposure to blue light[3] is sufficient to synchronise their circadian clock over a 24-hour day, older people require exposure to light that is richer in wavelengths (colours) – a light whose characteristics are those of sunlight.

‘This is the discovery of a new adaptive mechanism of the retina during ageing – enabling older subjects to remain sensitive to light despite yellowing of the lens. These findings are also clinically relevant, encouraging older people to have more exposure to daylight, which is richer in wavelengths, rather than artificial light, in order to avoid developing sleep disturbances or mood or metabolism disorders, for example. Finally, they offer new possibilities for the optimal personalisation of phototherapies/light therapies for older people‘, explains Claude Gronfier, Inserm researcher and last author of the study.

Regarding this last aspect, the research team is now looking at the quantity and quality of light necessary for each individual, and the best time for light exposure during the day, to prevent the development of sleep disorders and health problems more generally.

The research is being conducted in healthy subjects (children and adults), day and night workers, and patients (with sleep and biological rhythm disorders, genetic diseases, mood disorders and neurodegeneration)[4].

[1]In this study, the average age of the participants in the ‘older’ group was 59 years.

[2] https://www.has-sante.fr/jcms/c_1299994/troubles-du-sommeil-stop-a-la-prescription-systematique-de-somniferes-chez-les-personnes-agees

[3] The LED lights used are rich in blue light.

[4] https://www.crnl.fr/fr/page-base/groupe-sommeil-rythmicite-circadienne-lhumain-epidemiologie-populationnelle-recherche

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