Health consequences of electric lighting practices in the modern world: A report on the National Toxicology Program's workshop on shift work at night, artificial light at night, and circadian disruption

Abstract

The invention of electric light has facilitated a society in which people work, sleep, eat, and play at all hours of the 24-hour day. Although electric light clearly has benefited humankind, exposures to electric light, especially light at night (LAN), may disrupt sleep and biological processes controlled by endogenous circadian clocks, potentially resulting in adverse health outcomes. Many of the studies evaluating adverse health effects have been conducted among night- and rotating-shift workers, because this scenario gives rise to significant exposure to LAN. Because of the complexity of this topic, the National Toxicology Program convened an expert panel at a public workshop entitled "Shift Work at Night, Artificial Light at Night, and Circadian Disruption" to obtain input on conducting literature-based health hazard assessments and to identify data gaps and research needs. The Panel suggested describing light both as a direct effector of endogenous circadian clocks and rhythms and as an enabler of additional activities or behaviors that may lead to circadian disruption, such as night-shift work and atypical and inconsistent sleep-wake patterns that can lead to social jet lag. Future studies should more comprehensively characterize and measure the relevant light-related exposures and link these exposures to both time-independent biomarkers of circadian disruption and biomarkers of adverse health outcomes. This information should lead to improvements in human epidemiological and animal or in vitro models, more rigorous health hazard assessments, and intervention strategies to minimize the occurrence of adverse health outcomes due to these exposures.

Keywords: Artificial light at night (ALAN); Circadian disruption; Light at night (LAN); Shift work.

Figure 1

Regulation of circadian rhythms by internal and external cues. Light is the primary regulator of the master circadian clock found in the suprachiasmatic nucleus (SCN) of the brain. The SCN sends endocrine and neural signals to a variety of peripheral tissues to temporally coordinate their physiology and metabolism. The SCN also sends a signal to the pineal gland to produce the hormone melatonin during darkness at night. Melatonin can then convey signals back to the SCN, other parts of the brain, and peripheral tissues to help coordinate physiological functions and behaviors to approximate 24-hour days.

Figure 2

World map of artificial brightness as a ratio to the natural sky brightness (Falchi et al. 2016).

Figure 3

Nocturnal melatonin suppression for different light spectra plotted as a function of light level, where the spectral power distributions of various light sources used in previous published studies are weighted according to the CLA (x-axis). CLA is irradiance at the cornea weighted to reflect the spectral sensitivity of the human circadian system as measured by acute melatonin suppression, and is measured in units of spectrally weighted flux per unit area. The right ordinate (y-axis), labeled “circadian stimulus” (CS), is scaled to be proportional to the left ordinate, representing the relative amount of melatonin suppressed after exposure of the retina for 1 hour, ranging from 0.0 (no suppression) to a maximum of 0.7 (70% suppression).

Figure 4

Design considerations for epidemiological studies of shift workers. Shift work is a complex multi-dimensional exposure with a range of associated effects or exposures. The ideal study of shift work would capture, in addition to lifetime patterns of shiftwork, measurements of disturbed social, sleep, and dietary patterns; lifestyle aspects; and changes in light-at-night and sun exposure, which are potentially modified by chronotype. Potentially relevant exposures are measured by self-report (e.g., interviews or questionnaires), employer data, actigraphy, 24-hour recall logs, light sensors, urinary, blood, or fecal biomarkers, anthropometry, psychomotor vigilance tasks, and memory tests. Gray boxes represent methods to measure these complex exposures and intermediate outcomes; numbers in the related-exposures or intermediate-effects boxes relate to the corresponding type of “measurement methods” to assess the factors; (Δ = change, arrows indicate direction of change).

Figure 5

Schematic depicting animal models of shift work showing several exposure components acting via a number of putative pathophysiological mechanisms and resulting in a series of adverse chronic health outcomes. These models may help disentangle the relationships among exposure components in a systematic manner, and may help in assessing the impact of relevant variables on shift-work exposure outcomes and their underpinning mechanisms. Modifying factors may include age, sex, the nature of the shift-work exposure (e.g., fast vs. slow, forward vs. backward rotating), circadian phenotypes (e.g., morning types vs. evening types), and the nature of the background photic exposure onto which the shift-work-related light-dark cycle is imposed (e.g., long, summer-like photoperiods vs. short, winter-like photoperiods).