Artificial Light at Night- More is not Always Better

Artificial light at night disrupts circadian rhythms, affecting health and linking to diseases. Discover risks and tips to reduce nighttime light exposure.

[Barbara Harding is assistant professor at College of Population Health, University of New Mexico, Albuquerque, NM, USA, and associated researcher at ISGlobal.]

Electric light transformed our nights and societies, but at what cost? Artificial light at night disrupts natural cycles, impacting health and the environment.

The earth's rotation has created a consistent 24-hour light/dark cycle, which has profoundly influenced the evolution of organisms, including humans. We have adapted to this rhythmic environment by developing circadian clocks to keep physiological and behavioral processes in sync with this light/dark cycle. Our internal clocks regulate sleep, metabolism, hormone production, and countless other processes, ensuring optimal functioning and survival.

Artificial light at night (abbreviated ALAN) is the term given to describe light exposure from non-natural sources which occurs during the typically dark hours of the 24-hour day. For most of history to date, light came from flames, and daily life revolved around the natural light-dark cycle, dictated by sunrise and sunset. However, since the commercial use of electric lights in the 19^th Century, this has drastically changed. The amount of artificial light at night has been rapidly increasing and has resulted in wide-ranging light pollution.

Artificial light at night and its impacts have been steadily gaining more focus, starting with the discoveries about the molecular mechanisms controlling circadian rhythms by Jeffrey Hall, Michael Rosbash and Michael Young in the early 21^st century, which lead to their Nobel Prize in physiology in 2017. More recently, media attention on the topic has increased, as evident from a 2023 special issue in Science.

The hidden health risks of artificial light at night

Populations generally equate artificial light at night to increased safety or visibility, or as a symbol of industrialization and economic achievement. But importantly, we need to focus more on how artificial light at night impacts human health (and the health of our planet as a whole). Light exposure has direct consequences for circadian rhythms, which are integral to regulating countless biologic processes. While circadian rhythms are endogenous, they are synchronized daily by exogenous cues, of which light is the main cue regulating the central clock in the brain. Briefly, the way that light regulates circadian rhythms involves a particular —or blue light. Exposure to light, and particularly to blue light, sets off a cascade in the body which results in a suppression of melatonin production and an influence on circadian rhythms.

We observed that higher light exposure (both total and blue) was associated with negative cardiometabolic outcomes including: hypertension, as well as incident obesity, incident hemorrhagic stroke, and incident high cholesterol (total light only for high cholesterol

Given the potential misalignment of circadian rhythms as a result of improper timing of light cues, the examination of artificial light at night and subsequent population health is important. The ISGlobal’s circadian health group has focused on examining the connection between ALAN and cardiometabolic outcomes, given that prior evidence has suggested a link between artificial light at night, circadian disruption, and cardiometabolic disorders. In our analysis, we examined the association between outdoor artificial light at night exposure, measured from satellite images, on cardiometabolic risk in a population-based cohort in Catalunya. Specifically, we examined the relationship between light exposure (both total light exposure and specifically the blue light component of light, which is the most relevant for circadian disruption) and obesity, blood pressure, diabetes markers, and major cardiovascular outcomes including angina pectoris, myocardial infarction and others. We observed that higher light exposure (both total and blue) was associated with negative cardiometabolic outcomes including: hypertension, as well as incident obesity, incident hemorrhagic stroke, and incident high cholesterol (total light only for high cholesterol).

A potential link to cancer under investigation

Besides the focus on cardiometabolic health impacts of artificial light at night, there has been widespread coverage of the relationship between artificial light at night and other health conditions, including cancer. In fact, the International Agency for Research on Cancer (IARC) has recently announced that artificial light at night is recommended for evaluation as a potential carcinogen by the IARC monographs with high priority during the upcoming years.

Healthy habits for minimizing nighttime light exposure at home

There are small ways we can each take more action to improve our circadian health by reducing exposure to artificial light at night. This is not only something that can be addressed from a policy standpoint with alterations to the amount and types of light emitting sources in our outdoor communities during the biologic night. It is also something that we as individuals can take action to change. Indoor artificial light at night is also widespread, and children and adults alike are spending more and more time exposed to screens, including late at night and right before bed. Implementing healthy sleep hygiene practices, such as sleeping in a dark room, reducing screen usage in the hours before bedtime, reducing the amount of artificial light- or using blue light filtering settings close to bedtime, and having a regular bedtime routine and routine hours of sleep are ways to start.