Daylight heals, night light steals: The surprising link between light exposure and mental health

Background

Growing evidence indicates that healthy circadian rhythms play a pivotal role in mental health, with disrupted sleep cycles implicated in various psychiatric disorders. The suprachiasmatic nuclei in the hypothalamus in humans house the central circadian rhythm, which modulates physiological functions, behavior, cognition, as well as basic cellular functions. These circadian rhythms are, in turn, modulated by the patterns of light exposure during the day and at night.

Robust and stable circadian rhythms have evolved in biological systems under the predictable patterns of light exposure during the day and darkness during the night. In the modern age, humans spend most of the day indoors under artificial electric lighting, which is relatively dim compared to natural light during the day and bright at night. This disruption of the natural patterns of light exposure can subsequently disturb the circadian rhythms and result in psychiatric disorders and adverse mental health outcomes.

About the study

In the present study, the researchers used light-exposure data for over 86,000 individuals, recorded over seven days of light monitoring and actimetry to determine whether light exposure patterns during the day and at night were associated with various psychiatric disorders linked to disrupted circadian rhythms. The researchers hypothesized that an increase in light exposure during the day would be linked to better mood and a lower risk of psychiatric disorders, while an increase in night-time light exposure would conversely be linked to poorer moods and a higher risk of mental health disorders.

The data for the study was obtained from the United Kingdom Biobank, in which participants had completed a week-long light monitoring and actimetry assessment. Individuals with unreliable accelerometry, light, or sleep data were excluded. Light exposure was measured using a triaxial accelerometer worn on the wrist and containing a silicon photodiode light sensor.

An online mental health questionnaire was used to obtain information on mental health and psychiatric outcomes. The case/control psychiatric disorders were defined based on established guidelines. They included major depressive disorder, bipolar disorder including hypomania and mania, generalized anxiety disorder, post-traumatic stress disorder (PTSD), self-harm, and psychosis.

A multiple logistic regression model was used to evaluate the association between light exposure patterns during the day and night and the case/control outcomes. The analyses were also adjusted for covariates such as sex, age, ethnicity, physical activity levels, and employment. Sensitivity analyses examined whether the inclusion of shift workers in the dataset drove some of the observed associations. Other sensitivity analyses also examined the impact of clinical subgroups, urbanicity of the area of residence, and duration and efficiency of sleep on the observed associations.

Results

The findings reported that, as hypothesized, an increase in light exposure during the night was linked to a higher risk of generalized anxiety disorder, major depressive disorder, PTSD, bipolar disorder, self-harm, and psychosis. Furthermore, increased light exposure during the day, independent of the light exposure at night-time, was linked to a lower risk of self-harm, psychosis, PTSD, and major depressive disorder.

Additionally, these associations were significant even when the analyses were adjusted for covariates such as sociodemographic factors, physical activity levels, cardiometabolic health, and photoperiod. These associations were not only independent but also additive. Individuals in the quartile for brightest light exposure during the day were still at a higher risk of major depressive disorder if they had a higher light exposure at night-time. Conversely, those in the brightest light exposure quartile for night-time had a lower risk of major depressive disorder if they had a higher light exposure during the day.

The sensitivity analyses reported that the associations were consistent even after accounting for factors such as urbanicity, shift work, cardiometabolic health, and sleep quality. The risk of major depressive disorder and self-harm was 30% higher for individuals in the brightest light exposure quartile for night-time. In comparison, those in the brightest light exposure quartile during daytime had a 20% lower risk of self-harm and major depressive disorder.

Conclusions

To summarize, the study examined the association between light exposure patterns during the daytime and night-time and the risk of psychiatric disorders. The findings indicated that increased exposure to light during the night was associated with a higher risk of major depressive disorder, bipolar disorder, PTSD, psychosis, generalized anxiety disorder, and self-harm, while increased light exposure during the day was associated with a lower risk of psychiatric disorders. The study suggested that modulating light exposure patterns may provide a simple, non-pharmacological option for improving mental health outcomes.

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