OUR HEALTH ADVICE – Being more exposed to daylight reduces the risk of depression, anxiety, and other psychiatric illnesses. Conversely, too much nighttime light increases it. It’s a phenomenon that has never been measured on such a scale: researchers analyzed data collected from a British cohort of more than 80,000 adults, and calculated how exposure to daylight, and the darkness of night , has a decisive influence on our mental health.
90% of our time indoors
Easier said than done… Indeed, the researchers point out, modern man spends 90% of his time indoors, lit by electric lights that are much less powerful than those of the sun but much more powerful than those of the moon… Now, while it was once thought that only light of at least 1000 lux (the equivalent of a cloudy day) could influence our biological rhythm, we know that very dim light, like that of a simple candle, can be enough! At play in this association between light and mental health is the circadian clock, which regulates a great many biological functions. It is set to a rhythm of approximately 24 hours. But only “approximately”. Indeed, on average, in adults, it is set to a rhythm of 24 hours and 10 minutes. It must, therefore, be resynchronized every day, and light is the most powerful regulator.
Blue light
Nestled in the hypothalamus, the two “suprachiasmatic nuclei” that make up this clock are directly connected to cells in the retina sensitive to blue light, by pathways independent of visual perception. The two chief clockmakers innervate brain regions responsible for regulating appetite, sleep, body temperature, the release of hormones or neurotransmitters such as serotonin or dopamine, mood, cognition, etc. The retina also sends messages directly to these same brain regions. ” The circadian clock gives us the tempo, inducing a good mood during the day, but not at night since we have to sleep. When we reduce the light intensity during the day, we stimulate these structures less; if we illuminate them at night, we do not stimulate them at the right time, and all of this will artificially lower our moo, ” explains Claude Gronfier. The authors’ conclusion: ” Avoiding light at night (and in particular blue light emitted by LEDs and screens, editor’s note) and seeking it out during the day could be an effective non-pharmacological way to improve mental health. “
In a commentary accompanying the article, Jamie Zeitzer, a researcher at Stanford University, adds some nuances. First, there are limitations related to the device used to measure the brightness to which the volunteers were exposed: worn on the wrist, it does not accurately reflect the brightness to which the eye is exposed, especially if it is hidden by a sleeve or a sheet. ” The accuracy of this measurement remains sufficient for the question posed her, ” judges Claude Gronfier. As for the actimeter, it is a device commonly used in sleep medicine, simpler and less expensive than polysomnography, which consists of directly recording the electrical activity of the brain. ” This makes it very easy to determine what time a person went to bed, whether they took a nap during the day, had a fragmented night…,” explains Claude Gronfier. The actimeter has a slight tendency to overestimate the amount of sleep because one can be inactive without actually sleeping, for example, when watching television.Howeverw, it remains a good way to judge the quality of sleep in healthy subjects. However, it is a little less well validated in psychiatric patients, the elderly, or young subjects .
Longitudinal study
Jamie Zeitzer also notes that the study is not enough to assert that it is poor exposure to light that degrades mental health, and not conversely, degraded mental health that causes ” a disruption of sleep or sleep schedules (which means more light at night while awake) and a reduction in outdoor behavior (less light during the day) .” To decide, explains Claude Gronfier, it would be necessary to carry out a so-called “longitudinal” study, with volunteers equipped with light and movement sensors whose evolution of mental health would be monitored over the long term. However, everything ” suggests that the relationship is rather in the direction of an effect of exposure to light on mental hhealth” judges the chronobiologist. While further work is therefore ” necessary to better understand the dose-response relationship between nighttime light and mental health “, Jamie Zeitzer believes that exposing oneself to light at an untimely time ” can play a crucial role in worsening or reinforcing poor mental health.
