A study by Yale University in the United States indicates that nighttime lighting may increase the risk of hormone-related cancers

Lyu L. Light at night and cancer incidence: a GIS-based study in the US[D]. Yale University, 2024.

2024年，美国耶鲁大学公共卫生学院发表了一项研究——夜间光照与癌症发病率：基于美国GIS的研究。研究利用卫星夜间灯光数据与癌症发病率数据进行关联分析，发现夜间光照与激素相关癌症（例如女性乳腺癌、前列腺癌）呈正相关。研究结果与CIE PS 003:2025《干扰光与光污染立场声明》中“夜间光照会扰乱人体生物节律并造成健康风险”的内容高度一致合。

研究通过线性混合效应回归模型分析1996-2000年夜间光照（LAN）数据与2016-2020年癌症发病率的关联，发现LAN与全癌种发病率、激素相关癌症（女性乳腺癌相关系数0.42，校正后p=3.99E-13；前列腺癌相关系数0.5，校正后p=1.61E-06）呈正相关，与部分消化系统（结直肠癌、食管癌）和泌尿生殖系统癌症呈负相关，且最高LAN暴露水平的关联最显著，为后续流行病学和实验研究提供了线索。

I. What is nighttime lighting? Why does it matter?

Nighttime lighting refers to the artificial light we encounter at night, such as streetlights, billboards, and even the glow from mobile screens. This light can disrupt our circadian rhythm—a natural cycle that regulates sleep, eating, and hormone secretion. The darkness signal at night should put the body into "repair mode," but bright light may throw off the clock. Studies show that long-term exposure to LAN may reduce melatonin secretion, a hormone with anti-cancer properties, thereby increasing cancer risk.

Studies show that the regions with the highest nighttime illumination in the US are concentrated along the northeastern coast, while the western areas have relatively lower levels. The figure below illustrates the distribution of nighttime illumination across the United States from 1996 to 2000.

II. Data Sources and Processing

Data Type	Source	Key Parameters	Processing Method
Cancer Data	CDCCancer Statistics Database	26 types of cancer, age-adjusted incidence rate (per 10 persons), covering 44 states	Exclude states with non-compliant data / counties, apply log transformation standardization
LANData	DMSPSatellite Imagery (Version 4)	1996-2000 years, DN value ( 0-63 , higher value means stronger light)	Inter-satellite calibration (formula: DN adjusted =C0+C1×DN+C2×DN²), calculating the 5-year average
Confounding factors	2016-2020 years ACS survey	Insurance coverage rate, poverty rate, percentage of white population, education level, population density, etc.	incorporated into multivariate model adjustment

III. Nighttime Lighting and Cancer: What Are the Shocking Discoveries?

This study analyzed 26 cancer types and found a positive correlation between LAN and the overall incidence of "all cancers." That is, regions with stronger light exposure tend to have more cancer cases.

More specifically, LAN affects different types of cancer differently:

Increased risk of hormone-related cancers : For example, female breast cancer and prostate cancer are significantly positively correlated with LAN. This may be due to light disrupting hormonal balance. The figure below shows the relationship between LAN and breast cancer; incidence rates are higher in high-light areas (such as the Northeast).

Similarly, prostate cancer also shows a similar pattern, especially in states with high light intensity.

Some cancer risks are actually reduced : Surprisingly, LAN is negatively correlated with digestive system cancers such as colorectal cancer and esophageal cancer. This means that in areas with strong lighting, the incidence of these cancers is lower. The reasons may be complex; for instance, regions with higher urbanization often have better medical conditions, but the specific mechanisms require further research. The figure below illustrates the negative correlation pattern between LAN and colorectal cancer.

Non-Hodgkin lymphoma is also affected: LAN also shows a slight positive association with this lymphoma, suggesting that light exposure may act through the immune system.

IV. Why can nighttime lighting cause cancer? The scientific mechanism is simply explained as

The core mechanism lies in melatonin. When it is dark at night, our brain secretes melatonin, which not only aids sleep but also inhibits tumor growth. However, LAN acts like "fake daylight," deceiving the body into reducing melatonin production, thereby giving cancer cells an opportunity to thrive. Animal experiments have also confirmed that continuous light exposure accelerates tumor development. This is akin to sleeping with the lights on, preventing the body from receiving adequate repair.

V. What are the limitations of this study? What should we do?

It should be noted that this is an "ecological study," based on population data rather than individual data. In other words, it reveals trends but cannot directly prove that light exposure causes cancer in a specific person. Other factors such as diet and exercise may also confound the results. Additionally, satellite data cannot distinguish between indoor and outdoor lighting; more precise measurements are needed in the future.

尽管有局限，但LAN是一种可改变的风险因素。我们可以通过简单措施保护自己：比如睡前减少屏幕时间、使用遮光窗帘，或选择暖色调灯光。政府和社区也可以优化户外照明，减少光污染。

Light up health, starting with the night: The link between nighttime lighting and cancer reminds us that small changes in modern lifestyles can have profound impacts. This study offers new clues for cancer prevention, but scientific exploration continues. Let's cherish the darkness of the night more from today on and add a 'protective lock' to the health of ourselves and our families.

Source: Light Environment and Lighting Research Center