Sleep and circadian rhythm disturbances: plausible pathways to major mental disorders?, World Psychiatry, 12 January 2024

Of note has been the delineation of the molecular architecture of the core circadian clock, along with the revelation that the circadian system’s stability is fundamentally regulated by common environmental factors, such as the timing, intensity and spectrum of light exposure 4. It appears that there are specific brain circuits in mammals by which light regulates mood, learning and activity, which are not wholly dependent on mediation by the master circadian timekeeper.
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An intriguing finding from over 80,000 adults in the UK Biobank was that more exposure to artificial light at night was associated not only with increased rates of major depression, but also with an increased incidence of several other mental disorders, including bipolar disorder, generalized anxiety disorder and post-traumatic stress disorder, as well as with higher rates of self-harm behavior and psychosis-like experiences 4. As predicted on the basis of the evidence that day-time light exposure is the primary synchronizer of the circadian clock in mammals, as well as the success of bright light therapy in the treatment of mood disorders, more light exposure during the day was also associated with lower rates of mental disorders 4. Triangulation of evidence from animal models, experimental studies in humans, and epidemiology has provided strong evidence for a major role of daily light exposure to good mental health.

This was surprising:

The discovery that treatments such as selective serotonin reuptake inhibitors (SSRIs) may increase sensitivity to light, and thereby destabilize the circadian system in at-risk individuals, is a major concern 9.