2023-10-14 08:01:33
OUR HEALTH ADVICE – Being more exposed to daylight reduces the risk of depression, anxiety and other psychiatric illnesses. Conversely, too much nighttime brightness increases it.
This is a phenomenon that had 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 the night has a determining influence on our mental health.
In total, 86,772 adults (average age 62.5 years) members of the UK Biobank cohort wore an actimeter (device measuring motor activity) containing a light sensor on their wrist for a week for 7 days. Then, a year and a half later on average, they were questioned regarding various parameters, including their mental health. In the magazine Nature mental health , the authors show that greater exposure to nighttime light is associated with an increased risk of depression, anxiety, post-traumatic stress disorder, psychosis, bipolar disorder and self-harm behavior. Conversely, greater exposure to daylight reduces the risks. All this regardless of factors such as sociodemographic criteria, physical activity, sleep quality, cardio-metabolic health, living in an urban environment or having a shift job.
Avoiding light at night and seeking it out during the day might be an effective non-pharmacological way to improve mental health.
« It’s a very beautiful, large-scale piece of work.rejoices Claude Gronfier, chronobiologist at the Lyon Neuroscience Research Center (Inserm). The link between exposure to light and mental health had been measured on groups of a few dozen patients coming to the laboratory, never on such a large population, in real life, and coming from all over England ! »
The effect of light on mental health is far from minor: in the study, the quartile of the highest exposure to nighttime light increased the risk of depression by around 30%, while the quartile of the highest strong daytime light reduced it by 20%! « These associations were independent and added together », specify the authors. Sleeping in a dark room is good. But if you also expose yourself to natural light during the day, it’s even better!
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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 much less powerful than that of the sun, but much more than that of the moon… Now if we once thought that only light of at least 1000 lux (the equivalent of a cloudy day) might have an influence on our biological rhythm, we now know that very light thin, like that of a simple candle, can be enough!
At play in this association between light and mental health: the circadian clock, which regulates many biological functions. It is set to a rhythm of approximately 24 hours. But only “approximately”… In fact, in adults, it is on average set to a rhythm of 24h10mn. It must therefore be resynchronized every day, and light is the most powerful regulator.
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Blue light
Nestled in the hypothalamus, the two “suprachiasmatic nuclei” which make up this clock are in fact directly connected to cells of the retina sensitive to blue light, by pathways independent of visual perception. The two chief watchmakers innervate brain regions responsible for regulating appetite, sleep, body temperature, the release of hormones or neurotransmitters such as serotonin or dopamine, mood, cognition… The retina sends also directly send messages to these same brain regions. “ The circadian clock sets the pace for us, 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 this will artificially lower the mood. », explains Claude Gronfier. Conclusion of the authors: « Avoid light at night (and in particular the blue light emitted by LEDs and screens, editor’s note) and seeking it during the day might be an effective non-pharmacological way to improve mental health. »
In a commentary accompanying the article, Jamie Zeitzer, researcher at Stanford University, provides some nuances. First, limits are linked to the device used to measure the brightness to which the volunteers were exposed: worn on the wrist, it does not exactly reflect the brightness to which the eye is exposed, especially if it is hidden by a sleeve or sheet. « The precision of this measurement remains sufficient for the question posed here. », judge Claude Gronfier. As for the activity meter, 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, etc.explains Claude Gronfier. The activity monitor tends to overestimate the amount of sleep a little because the’we can be inactive without sleeping, for example when we watch television. But it remains a good way to judge the quality of sleep in healthy subjects. On the other hand, it is a little less well validated in psychiatric patients, elderly subjects, or young subjects. »
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Longitudinal study
Jamie Zeitzer also notes that the study is not enough to assert that it is poor exposure to light which degrades mental health, and not conversely poor mental health which causes « disruption of sleep or sleep schedules (meaning more light at night while awake) and reduction in outdoor behaviors (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 followed over the long term. However, everything « suggests that the relationship is more in the direction of an effect of exposure to light on mental health », judges the chronobiologist. If other work is therefore « necessary to better understand the dose response relationship between night light and mental health »the fact remains, believes Jamie Zeitzer, that exposing yourself to light at the wrong time “ can play a crucial role in worsening or reinforcing poor mental health ».
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