In a recent study published within the journal Nature Mental Health, a global team of researchers conducted a large-scale, cross-sectional evaluation of sleep, light exposure, mental health, and physical activity to know the association between light exposure throughout the day and night and the danger of self-harm and psychiatric disorders.

Study: Day and night light exposure are related to psychiatric disorders: an objective light study in >85,000 people. Image Credit: Gorodenkoff / Shutterstock

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 within the hypothalamus in humans house the central circadian rhythm, which modulates physiological functions, behavior, cognition, in addition to basic cellular functions. These circadian rhythms are, in turn, modulated by the patterns of sunshine exposure throughout the day and at night.

Robust and stable circadian rhythms have evolved in biological systems under the predictable patterns of sunshine exposure throughout the day and darkness throughout the night. In the fashionable age, humans spend many of the day indoors under artificial electric lighting, which is comparatively dim in comparison with natural light throughout the day and vibrant at night. This disruption of the natural patterns of sunshine exposure can subsequently disturb the circadian rhythms and lead to psychiatric disorders and antagonistic mental health outcomes.

Concerning the study

In the current study, the researchers used light-exposure data for over 86,000 individuals, recorded over seven days of sunshine monitoring and actimetry to find out whether light exposure patterns throughout the day and at night were related to various psychiatric disorders linked to disrupted circadian rhythms. The researchers hypothesized that a rise in light exposure throughout the day could be linked to higher mood and a lower risk of psychiatric disorders, while a rise in night-time light exposure would conversely be linked to poorer moods and the next risk of mental health disorders.

The information for the study was obtained from the UK Biobank, during which participants had accomplished 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 internet mental health questionnaire was used to acquire 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 guage the association between light exposure patterns throughout the day and night and the case/control outcomes. The analyses were also adjusted for covariates akin to sex, age, ethnicity, physical activity levels, and employment. Sensitivity analyses examined whether the inclusion of shift employees within the dataset drove a few of the observed associations. Other sensitivity analyses also examined the impact of clinical subgroups, urbanicity of the world of residence, and duration and efficiency of sleep on the observed associations.

Results

The findings reported that, as hypothesized, a rise in light exposure throughout the night was linked to the next risk of generalized anxiety disorder, major depressive disorder, PTSD, bipolar disorder, self-harm, and psychosis. Moreover, increased light exposure throughout the day, independent of the sunshine exposure at night-time, was linked to a lower risk of self-harm, psychosis, PTSD, and major depressive disorder.

Moreover, these associations were significant even when the analyses were adjusted for covariates akin to sociodemographic aspects, physical activity levels, cardiometabolic health, and photoperiod. These associations weren’t only independent but additionally additive. Individuals within the quartile for brightest light exposure throughout the day were still at the next risk of major depressive disorder in the event that they had the next light exposure at night-time. Conversely, those within the brightest light exposure quartile for night-time had a lower risk of major depressive disorder in the event that they had the next light exposure throughout the day.

The sensitivity analyses reported that the associations were consistent even after accounting for aspects akin to urbanicity, shift work, cardiometabolic health, and sleep quality. The chance of major depressive disorder and self-harm was 30% higher for people within the brightest light exposure quartile for night-time. Compared, those within 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 throughout the daytime and night-time and the danger of psychiatric disorders. The findings indicated that increased exposure to light throughout the night was related to the next risk of major depressive disorder, bipolar disorder, PTSD, psychosis, generalized anxiety disorder, and self-harm, while increased light exposure throughout the day was related to a lower risk of psychiatric disorders. The study suggested that modulating light exposure patterns may provide a straightforward, non-pharmacological option for improving mental health outcomes.