Artificial light at night, stress, and cardiovascular health Original paper

This review reported that greater exposure to artificial light at night can disrupt stress hormones and negatively affect cardiovascular health.

The effect of artificial light at night (ALAN) on stress and cardiovascular health outcomes.

Adults, including some night shift workers.

This narrative review summarized findings from various human studies that evaluated the effect of ALAN on heart rate, blood pressure, the autonomic nervous system, cortisol, and carotid artery intima-media thickness (a marker of subclinical atherosclerosis burden).^[1]

Despite a few conflicting findings, most of the human studies reported that greater ALAN is associated with higher blood pressure, heart rate, autonomic nervous system activity, and carotid artery intima-media thickness, lower day-night heart rate variability, and disrupted glucocorticoid activity.

Generally, associations seemed to be dependent on duration and dose (i.e., longer exposure to brighter ALAN was associated with worse outcomes), but potential confounders, such as noise and participant stress level (i.e., urban vs. suburban populations, daily routines/behavior), couldn’t be fully ruled out.

Light pollution (or ALAN) has been suggested as one of the most broadly threatening pollutants today because of its range of influence, from microbes to mammals to ecosystems, and its biological effects on behavior, physiology, and fitness.^[4] The Earth’s artificial light area and brightness has been increasing by 2% per year in recent decades^[5] and the threats to human health have been steadily increasing with intensified urbanization and industrialization.^[6] Indirect threats, such as changes in environment and ecology^[7] (e.g., animal migration patterns^[8] and pollination^[9]) and direct threats, such as increased rates of disease or disorder (e.g. cardiometabolic disease, sleep disruption),^[10], are increasingly reported.^[11]

ALAN can come from outdoor (e.g., car, street, and building lights) or indoor sources (e.g., household lights and screens) and tends to surpass recommended maximum levels of light exposure for humans at night.^[10][12] Because outdoor sources and extent of exposure to ALAN usually aren’t able to be controlled by the individual, the recommendations from the Second International Workshop on Circadian and Neurophysiological Photometry in 2019 focused on indoor environments and can be seen in the figure below, along with light intensity levels of common sources and spaces. The recommendations also emphasize “stable and regular daily light–dark cycles ... to reinforce good alignment of circadian rhythms, which may further benefit sleep, cognition, and health.”^[12] It’s also worth mentioning that the lower limit of light pollution in residential areas of most developed countries is defined as light intensities of 10 lux^[13] and it is estimated that 83% of the world’s population lives with light pollution.^[14]

Light exposure recommendations and levels of common sources and spaces

*Data from Brown et al. (2022)^[12], Gaston et al. (2013)^[15], and Wood et al. (2013).^[16]

Several studies have associated ALAN exposure with human health issues, such as cardiovascular disease, but the correlational nature and potential confounding of these findings should be emphasized because they limit the understanding of causal relationships — especially in the context of something as pervasive and prevalent as artificial light.^[10] Thus, research must explore the mechanistic understanding of the association between ALAN exposure and human health, which likely has to do with altered circadian rhythm.

The circadian system coordinates physiological, mental, and behavioral rhythms to various external and internal cues, with the most influential cue being the 24-hour light-dark cycle.^[17] The central pacemaker of the circadian system is the suprachiasmatic nucleus (SCN) found within the hypothalamus, which plays a role in the synchronization of almost all physiological activity (e.g., blood pressure, temperature, mental alertness).^[18] It is directly connected to the retina in the eye to ensure synchronization with the 24-hour light-dark cycle and projects to various internal “clocks”. Internal or local clocks are run by transcriptional-translational feedback loop (TTFL) mechanisms, which are essentially on/off switches that are triggered by certain cues, such as level of light exposure or quantity and timing of food intake, and generally oscillate between two states (e.g., wake/sleep, feed/fast).

ALAN is known to disrupt the natural light cues that constantly realign biological rhythms via the SCN, causing circadian rhythm abnormalities that may lead to sleep and mental disorders, immune impairment, cancer, and endocrine and cardiovascular disease.^[19] The primary mechanism seems to be via the inhibition of melatonin secretion and downregulation of circadian-related genes involved in TTFLs that are both closely related to wake/sleep behavior. Altered wake/sleep behavior can go on to influence sleep architecture, fatigue, heart rate, oxidative damage, inflammation, fat storage, insulin resistance, and more.^[20]

Factors that influence the circadian rhythm

Adapted from Poggiogalle, Jamshed & Peterson, 2018.

In the specific case of cardiovascular disease, ALAN seems to inhibit melatonin secretion, which reduces mitochondrial recycling (mitophagy) and efficiency (e.g., fusion),^[21] and circadian clock gene expression that disregulates the circadian rhythm and can lead to systemic inflammation and oxidative stress. Given that several cardiovascular parameters, like blood pressure and heart rate, are modulated by the circadian system and show clear 24-hour rhythms, it is not surprising that exposure to a cue that alters the rhythm will influence the cardiovascular parameter, especially when the external cue conflicts with internal clocks.^[22] However, it should be noted that ALAN is not the only factor involved in circadian disruption, as exposure to cues like activity, temperature, or food intake outside of circadian rhythms could also contribute to circadian rhythm abnormalities and any associated health issues.

Overall, current studies seem to suggest that ALAN can put stress on the body by providing conflicting cues to the circadian system that dysregulate rhythmic biological functions such as heart rate and cellular recovery.^[19] Generally, acute stress is relatively harmless, as the body can recover from or adapt to it.^[23][24] However, if the cue is consistent enough, it may be considered a chronic stressor that manifests as a disease or disorder, such as a compromised cardiovascular system. This appears to be the case for ALAN.^[25] However, there is a small amount of human research in this field that makes it difficult to draw strong conclusions. The difficulty is magnified when determining causality because there are so many variables involved in a disease or disorder, its links to circadian biology, and ALAN exposure. Although ALAN has been associated with some adverse health outcomes, the dose and duration of exposure, other circadian cues (e.g., timing of meals and physical and mental exertion) and associated behaviors/habits (e.g., doing more extracurricular activities, watching screens before bed) and any other associated variables should be kept in mind when evaluating the magnitude of effect of ALAN on health outcomes.

The general recommendation is to maintain consistent lifestyle habits and behavioral patterns that align with the circadian rhythm for better health, including timing of wake/sleep, feed/fast, and exertion/recovery, as well as light exposure, because life is full of events that will periodically disrupt the natural rhythm.^[26] If consistency is the norm, a few acute stresses of ALAN or a late night snack should be straightforward for the body to recover from and shouldn’t lead to cardiovascular disease. For people with cardiovascular disease, ALAN management fits into stress reduction, along with sleeping well and avoidance of tobacco and alcohol within a bigger picture prevention/treatment plan that includes exercise, weight management, and dietary management.^[27]

Although the circadian system is often investigated in the context of animal or in vitro research, much of circadian physiology and the core molecular clock (e.g., the SCN and transcription-translation feedback loops) is conserved across organisms, which translates to more confidence in the generalizability of the results to humans.^[2] In fact, a review of animal studies reported that failing to account for time of day and failing to consider how the timing of an experimental test aligns with the circadian rhythm of a model organism (e.g., getting a rat to run in the middle of the night according to its circadian rhythm) can compromise the replication of findings and reduce their value when extrapolating to humans.^[3]