Research shows that exposure to bright light at night raises mortality risk, particularly from heart disease and diabetes, but seeking daylight could lower these risks and promote longevity.
Study: Brighter nights and darker days predict higher mortality risk: A prospective analysis of personal light exposure in >88,000 individuals. Image Credit: VesnaArt / Shutterstock
In a recent study published in the journal Proceedings of the National Academy of Sciences, researchers investigated whether personal day and night light exposure and circadian rhythm disruption could predict the risk of mortality.
They found that exposure to brighter daytime light reduced mortality risk. In comparison, exposure to brighter nighttime light and disrupted circadian rhythms (low amplitude, early or late phase) increased mortality risk, particularly for cardiometabolic causes, which showed stronger associations than other causes.
Background
Disruption of the circadian rhythm is linked to various adverse health outcomes, including an increased risk of premature mortality. Nighttime light exposure shifts the timing and weakens the signal of the circadian pacemaker in the hypothalamus, leading to circadian disruption.
Animal studies show that exposure to light at night causes premature death, and human populations exposed to more night light, such as shift workers and evening types, have higher mortality risks.
While night light disrupts circadian rhythms, daylight stabilizes and strengthens them, potentially reducing sensitivity to night light and protecting against circadian-related health issues.
Studies suggest that daylight exposure lowers risks of all-cause mortality and cardiovascular diseases, though they often rely on self-reports or satellite data. Circadian disruption, particularly from irregular light exposure, is associated with cardiometabolic disorders like hypertension, diabetes, and heart disease.
However, there is a dearth of large-scale studies on personal light exposure and cardiometabolic mortality risks. Therefore, researchers in the present study investigated the potential association between day and night light, circadian rhythms, and mortality risk using ~13 million hours of personal light exposure data obtained from light sensors.
About the study
In the present study, ~89,000 participants from the United Kingdom (UK) Biobank wore wrist devices to track personal light exposure data over about seven days.
Light exposure data were collected at 100 Hz using a light sensor sensitive to human vision, and profiles were created based on half-hour intervals across the 24-hour day.
Participants with insufficient data or device issues were excluded, leaving 88,905 individuals. The mean age of the participants was 62.4 years, 56.9% of them were female, and 97% were White.
Mortality data, including those of all-cause, cardiometabolic-cause, and other-cause mortality, were collected from the National Health Service (NHS) Digital (England) and NHS Central Register (Scotland) over a mean follow-up period of eight years.
Baseline covariates like ethnicity, income, smoking, and physical activity were considered, along with photoperiod and sleep data. Cox proportional hazards models were used to estimate mortality risk, and light exposure percentiles were divided into day and night groups.
Circadian phase and amplitude were modeled using a computational approach, predicting the circadian disruption based on personal light exposure data. Z-scores of amplitude values and phase variability were included in the models.
Results and discussion
A total of 3,750 all-cause deaths and 798 cardiometabolic deaths were recorded. Exposure to brighter night light was found to be associated with increased all-cause mortality risk (adjusted hazard ratio [aHR] range: 1.15 to 1.34), with stronger associations for cardiometabolic mortality (aHR range: 1.22 to 1.46), and other-cause mortality (aHR range: 1.13 to 1.15).
On the other hand, exposure to brighter daylight was found to correlate with lower risks of all-cause mortality (aHR range: 0.66 to 0.90) and cardiometabolic mortality (aHR range: 0.61 to 0.84).
Individuals in higher night light percentiles showed significantly elevated risks compared to those in the darkest percentiles (0 to 50th). Additionally, lower circadian amplitude and both early and late circadian phases predicted higher mortality risks.
The study also found that short sleep (less than six hours) partially attenuated the association between night light and all-cause mortality. However, the impact of daylight exposure remained significant, regardless of sleep duration or efficiency.
Overall, the findings suggest that avoiding nightlight and seeking daylight may promote better health and longevity, offering an accessible and cost-effective recommendation for potentially improving well-being.
However, the study is limited by its reliance on only one week of light data, the computational model based on younger adults, and the lack of data on individual light sensitivity. There is also a limitation regarding the cohort’s predominantly white ethnicity, which may affect generalizability.
Conclusion
In conclusion, the study suggests that maintaining darkness during late night and early morning, when circadian rhythms are most sensitive, and seeking bright light during the day can potentially enhance circadian function. This is especially important for those at risk of circadian disruption, such as in intensive or elderly-care settings.
Across the general population, minimizing night light and maximizing daylight exposure may reduce disease burden, particularly cardiometabolic diseases, and potentially increase longevity.
How important circadian rhythm is for promoting our health: a study of ~89,000 @UK_biobank participants with 13 million hours of light sensorshttps://t.co/yf84XtD0YL @PNASNews @circadian247
“Minimizing night light, maximizing day light, and keeping regular light–dark patterns… pic.twitter.com/yFuDvEh5Gy— Eric Topol (@EricTopol) October 15, 2024
