Brighter Nights and Darker Days Linked to Higher Mortality Risk: New Public Health Insights

Key Takeaways

• Brighter night light exposure and darker daytime light are linked to higher all-cause and cardiometabolic mortality risks.
• This study tracked over 88,000 individuals, collecting 13 million hours of personal light data with wearable sensors.
• Maintaining bright daytime light and minimizing night light exposure may promote better cardiometabolic health and longevity.

Introduction

Light is more than a tool to illuminate our world—it regulates our internal clocks and rhythms, playing a pivotal role in health and disease. A new study published in PNAS has shed light on how exposure to brighter nights and darker days may elevate the risk of premature mortality, particularly from cardiometabolic causes. Researchers collected over 13 million hours of personal light exposure data from 88,000 individuals in the UK Biobank cohort, revealing strong associations between disrupted circadian rhythms and higher death rates.

Understanding Circadian Rhythms and Mortality Risk

Humans operate on an internal 24-hour cycle, known as the circadian rhythm, which is primarily regulated by light exposure. During the day, exposure to bright light stabilizes circadian rhythms, enhancing overall health. Conversely, night-time light exposure can disrupt these rhythms, leading to adverse health outcomes.

This study focused on understanding how variations in personal light exposure patterns during day and night might predict the risk of mortality. The researchers employed wrist-worn light sensors to track exposure levels and modeled how this light influenced the participants’ circadian rhythms over time. The analysis revealed significant findings:

• Brighter nights were associated with a 15-34% higher all-cause mortality risk.
• Darker days corresponded to a lower amplitude of circadian rhythms, which was linked to higher mortality.
• Cardiometabolic deaths, including heart disease, stroke, and diabetes, were more strongly associated with disrupted light exposure patterns than deaths from other causes.

Study Findings: Day and Night Light Exposure

One of the central conclusions of the study was the clear relationship between brighter night light exposure and an elevated risk of all-cause mortality. For individuals in the highest exposure percentile (the brightest 10% of night light), the risk of premature death from cardiometabolic causes increased by as much as 46% compared to those with low night light exposure. On the other hand, brighter daytime light exposure had the opposite effect, lowering mortality risks by up to 34% for those in the highest percentile.

Key Light Exposure Patterns

• Night Light: Exposure to bright night-time light (between 1:00 AM and 6:00 AM) significantly disrupted circadian rhythms, leading to increased mortality risk. The highest risk was observed between 2:30 AM and 3:00 AM, when cardiometabolic mortality risk was 56-67% higher in individuals exposed to bright night light.
• Day Light: Participants who were exposed to brighter daylight between 8:30 AM and 6:00 PM exhibited lower mortality risks. Higher day light exposure enhanced circadian amplitude, helping protect against cardiometabolic disease.

Implications for Public Health

The study’s findings have crucial public health implications, particularly as circadian rhythm disruption becomes a more prominent health issue in the modern world. In urbanized environments, artificial lighting is pervasive, and the exposure to artificial light at night is more common. Shift workers, city dwellers, and individuals with fragmented activity patterns are among the groups most likely to experience circadian disruption.

Protecting Circadian Health

Public health interventions that promote circadian health could help reduce the burden of cardiometabolic diseases, which are significant contributors to global mortality. Recommendations to minimize light exposure at night and prioritize bright light during the day could be simple, cost-effective strategies to improve public health outcomes. Specifically:

• Encouraging urban planners to limit outdoor light pollution and ensure dark night-time environments in residential areas.
• Raising awareness about the importance of reducing screen time before bed and using blackout curtains to limit indoor light exposure during sleep.
• Promoting exposure to natural daylight, particularly in workplaces and schools, where artificial lighting often dominates the environment.

Mechanisms Behind Light Exposure and Health Outcomes

The mechanisms by which light affects mortality risk are linked to its impact on the circadian pacemaker, located in the brain’s hypothalamus. Night-time light exposure suppresses the amplitude of the circadian rhythm, leading to metabolic disruptions that contribute to diseases such as diabetes, obesity, and cardiovascular disease. Disrupted circadian rhythms can also influence hormone secretion, sleep patterns, and overall metabolism.

Cardiometabolic Health

The study found that the association between light exposure and mortality was particularly strong for cardiometabolic diseases. These conditions are heavily influenced by lifestyle factors such as diet, physical activity, and, as this study demonstrates, light exposure. Brighter night-time light weakens circadian regulation of key metabolic processes, leading to insulin resistance, higher blood pressure, and poor heart health over time.

Conclusion

This large-scale study underscores the critical importance of maintaining regular light-dark cycles to support health and longevity. With modern society becoming increasingly exposed to artificial light at night, understanding and mitigating the effects of disrupted circadian rhythms has become a public health priority.

By encouraging more exposure to daylight and minimizing night light exposure, public health policies can help reduce the burden of cardiometabolic diseases and improve overall well-being. For individuals, small changes like increasing daytime light exposure and creating a darker sleeping environment could yield substantial long-term health benefits.

The featured image for this article is gathered from PickPik under a Public Domain License.