Exposure to light at night disrupts circadian rhythms that are essential to human health and well-being, while also significantly contributing to the prevalence of mood disorders in modern societies, according to a study by US researchers.

Exposure to light at night is associated with risk of psychiatric disorders, especially mood and behavioural disorders

According to the authors, a score of mood disorders, including depression, bipolar disorder and post-traumatic stress disorder, involve disturbances in sleep and circadian rhythms that are often triggered by irregular light exposure. For example, using electronics at night can inadvertently phase-shift the circadian rhythm and separate it from the natural light and dark cycle.

Specifically, exposure to light at night, even in low amounts, can significantly suppress melatonin production, which can disrupt the body’s internal clock. Circadian rhythms affect brain systems that regulate mood and control emotions, neurotransmitters and stress response systems. They also regulate the release of glucocorticoids, stress hormones from the adrenal glands, which usually peak in the morning and decline during the day. Proper regulation glucocorticoids is crucial for stress management and its disruption is associated with various mood disorders, with elevated cortisol levels (hypercortisolemia) observed in some patients with severe depression.

One of the most common effects of exposure to night-time lighting is sleep disruption.

Sleep disturbances are an important factor contributing to the development and maintenance of mood disorders. Subjects who used an electronic reader (30-50 lux) within 4 hours before bedtime experienced an increase in falling-asleep time and a decrease in morning alertness. These symptoms were related to a marked suppression of melatonin. Sleep and emotion regulation are closely related, and sleep disruption is therefore one of the mechanisms by which night-time light contributes to impaired mood.
The study suggests that in addition to disrupting sleep, night-time light may indirectly affect mood by interfering with processes such as brain plasticity, neurotransmission, hormone secretion and gene expression. These processes are controlled by circadian rhythms and are sensitive to changes in the daylight cycle.