Healthy sleep and circadian rhythms (the natural, internal processes that regulate our bodily functions with the environment and repeat roughly every 24 hours) are critical to mental well-being. Many psychiatric disorders are characterized by sleep-wake disturbances. In humans, the suprachiasmatic nucleus (SCN) is considered a central pacemaker, sending information to other parts of the brain and organs to synchronize circadian cycles (Hastings et al., 2019). It is the main entrance to the SCN light (Tahkamo et al., 2019). In modern and industrialized societies, artificial lighting and days spent mostly indoors challenge this natural rhythm and potentially worsen mental health outcomes (Walker et al., 2020).
In this current study, Burns and colleagues (2023) used data from over 85,000 adults to examine 24-hour light exposure and examine the relationship between light patterns and psychiatric disorders, adjusting for age, sex, and lifestyle. They assumed it increased daylight is associated with a reduced risk of psychiatric disorders and elevated moodand increased nighttime exposure is associated with increased risk of psychiatric disorders and worse mood. Understanding these relationships can inform interventions aimed at promoting mental health by maintaining healthy circadian rhythms.
Burns and colleagues (2023) hypothesized that increased nighttime light exposure would be associated with increased risk of psychiatric disorders and worse mood.
Methods
Data from the prospective general population cohort of the UK Biobank (a large and extensive biomedical database containing genetic, lifestyle and health data obtained in the United Kingdom) was used to examine the relationship between daily light exposure and risk for a range of psychiatric disorders.
Light exposure was taken from a wrist-worn actigraphic watch with an internal light sensor. Participants wore an actigraphic watch for seven days. Statistical analysis found that the hours from 7:30 am to 8:30 pm are daytime, and the hours from 0:30 am to 6:00 am are night time.
Psychiatric outcomes were assessed in an online mental health survey. According to the Diagnostic Manual of Mental Disorders (DSM), each participant had major depressive disorder (MDD), generalized anxiety disorder (GAD), bipolar disorder, post-traumatic stress disorder (PTSD), psychosis, and self-harm. Associations between daytime and nighttime light exposure and psychiatric outcomes were examined using multiple logistic regression. The effects of age, sex, ethnicity and photoperiod (the time from sunrise to sunset) were taken into account. Future models also considered the effects of employment and physical activity.
Results
Analyzes included 86,631 participants (mean age = 62.4 years, 57% women) with complete data on actigraphy measures, sleep, light exposure, and all identified covariates. Average daylight exposure was 1,380 lux and nighttime exposure was 24 lux. Luxuries are uniform dimensions of lighting, equal to one lumen per square meter. For example, direct sunlight typically provides about 32,000 to 100,000 lux, a full moon in a clear sky provides about 0.05 to 0.3 lux, and artificial lighting in a living room typically provides 50 to 100 lux.
Greater exposure to nighttime light was associated with higher risk of MDD, self-harm, GAD, PTSD, and psychosis.. Bipolar disorder risk was not significantly associated with light exposure, except for those in the brightest nighttime light quartile. Higher nighttime light exposure was also associated with lower health scores.
Greater exposure to daylight was associated with reduced risk of MDD, self-harm, PTSD, and psychosis.. GAD and bipolar disorder were not associated with daytime light exposure.
In addition to, greater nighttime light exposure was associated with more co-occurring psychiatric disordersto work greater exposure to daylight was associated with fewer psychiatric disorders. Moreover, while there was no interaction between daytime and nighttime light exposure in its associations with any psychiatric outcomes, increased daylight exposure moderated the association between nighttime light exposure and poorer well-being.
Burns and colleagues performed a series of sensitivity analyzes to further confirm these findings:
- Excluding participants who reported shift work and had a mental health disorder did not change the results.
- The findings were independent of sleep duration, sleep efficiency, and type of residence (urban vs. rural), except for the association between greater nighttime light exposure and psychosis, which became nonsignificant when sleep characteristics were considered.
- Body mass index, systolic blood pressure, and diabetes status did not influence any outcome.
In a sample of more than 85,000 adults, daily light exposure patterns were associated with psychiatric disorders and mood independent of age, sex, ethnicity, photoperiod, occupation, physical activity, sleep quality, and lifestyle.
Results
In this study, Burns and colleagues observed that Objectively measured light exposure in free-living conditions was associated with risk of psychiatric disorders in a large population of over 85,000., regardless of age, sex, ethnicity, photoperiod, occupation, and physical activity. More light at night was associated with increased risk of MDD, self-injurious behaviors, PTSD, psychosis, and GAD, in addition to poorer well-being. Conversely, greater exposure to daylight was associated with lower risk of MDD, self-injurious behaviors, PTSD, and psychosis, as well as greater well-being.
Avoiding light at night and seeking light during the day may be a simple and cost-effective non-pharmacological way to improve mental health.
Strengths and limitations
The study analyzes data from a significant sample of over 85,000 participants strong statistical power to detect associations between light exposure and psychiatric outcomes. In addition, to ensure meaningfulness and validity, as well as to avoid any bias and confounding findings, the analyzes were controlled for a number of relevant confounding variables, including age, sex, ethnicity, photoperiod, occupation and physical activity. to strengthen reliability from the findings, comprehensive sensitivity analyzes were performed confirming independence from shift work, urbanism, sleep quality, and cardiometabolic measures. In addition, Light exposure was recorded objectively minimizing self-report bias using a wearable actigraphic watch.
However, some limitations should be noted:
- First of all, the study was one cross-sectional design, thus preventing any causality from being established and allows for the possibility of reverse causation.
- Furthermore, although exposure to light is measured objectively, accuracy concerns It has been raised with the use of wrist-worn devices that exposure to light and the inability to discern true darkness or device coverage can lead to potential measurement error.
- Importantly, exposure data and psychiatric outcomes were not collected concurrentlydue to the recruitment schedule, may introduce biases related to changes in light exposure patterns over time.
In industrialized societies, people spend about 90% of the day indoors under artificial lighting, which is typically less bright during the day and brighter at night than outside light exposure.
Implications for practice
Overall, this article examines the effects of light exposure on psychiatric outcomes in a large adult population. The authors were motivated by evidence of the effects of daytime and nighttime light exposure on the circadian system, whose disruption has been linked to many psychiatric disorders. The results of this study have several implications for practice:
- Light therapy interventions
- Encouraging individuals to increase their exposure to bright natural light during the day and minimize exposure to artificial light at night may be a simple and cost-effective adjunctive treatment for a number of psychiatric disorders.
- Recommendations for optimizing light exposure can be easily integrated into clinical practice.
- In addition, light therapy offers a non-pharmacological approach to improving mental health outcomes that targets the underlying biological mechanisms of some psychiatric disorders.
- Transdiagnostic treatment
- The observed association between light exposure and psychiatric outcomes suggests that interventions targeting circadian rhythms may have transdiagnostic effects in several mental health conditions.
- Addressing common underlying pathophysiological mechanisms shared by different disorders may improve treatment strategies and outcomes.
- recommended for environmental Modification
- Optimizing exposure to light may go beyond individual behavioral changes to include environmental changes.
- Designing living and working spaces to maximize access to natural light during the day and minimize exposure to artificial light at night can support the mental health and well-being of individuals in industrialized societies.
- Interdisciplinary cooperation
- The results of this study suggest that collaboration between health care providers, architects, urban planners, and lighting designers can facilitate the development of environments that support healthy light exposure patterns.
- Integrating circadian rhythm principles into architectural and urban planning projects can promote mental health and well-being at the population level.
Overall, incorporating recommendations for optimizing light exposure into clinical practice and environmental design has the potential to improve mental health and enhance quality of life among diverse populations.
This study highlights the need to incorporate recommendations for optimizing light exposure into clinical practice and environmental design to improve mental health globally.
Statement of interest
No conflict of interest can be declared.
Connections
Primary paper
Burns, AC, Windred, DP, Rutter, MK, Olivier, P., Vetter, C., Saxena, R., Lane, JM, Phillips, AJK & Cain, SW (2023). Daylight and nighttime light exposure are associated with psychiatric disorders: an objective light study in >85,000 people. Nature Mental Health 1, 853–862.
Other references
Hastings, MH, Maywood, ES, & Brancaccio, M. (2019). The mammalian circadian timing system and the suprachiasmatic nucleus as its pacemaker. Biology, 8(1), 13.
Tähkämö, L., Partonen, T., & Pesonen, AK (2019). A systematic review of the effects of light on the human circadian rhythm. Chronobiology International, 36(2), 151-170.
Walker, WH, Walton, JC, DeVries, AC, and Nelson, RJ (2020). Circadian rhythm disorders and mental health. Translational Psychiatry, 10(1), 1-13.