Naturalistic Intensities of Light at Night: A Review of the Potent Effects of Very Dim Light on Circadian Responses and Considerations for Translational Research

Abstract

In this review, we discuss the remarkable potency and potential applications of a form of light that is often overlooked in a circadian context: naturalistic levels of dim light at night (nLAN), equivalent to intensities produced by the moon and stars. It is often assumed that such low levels of light do not produce circadian responses typically associated with brighter light levels. A solid understanding of the impacts of very low light levels is complicated further by the broad use of the somewhat ambiguous term "dim light," which has been used to describe light levels ranging seven orders of magnitude. Here, we lay out the argument that nLAN exerts potent circadian effects on numerous mammalian species, and that given conservation of anatomy and function, the efficacy of light in this range in humans warrants further investigation. We also provide recommendations for the field of chronobiological research, including minimum requirements for the measurement and reporting of light, standardization of terminology (specifically as it pertains to "dim" light), and ideas for reconsidering old data and designing new studies.

Keywords: circadian; dim light; entrainment; nLAN; parametric effects; physiology; translation.

Figure 1

Distribution of light intensities referred to as “dim light” in human circadian research papers. Light intensities were converted into melanopic lux [using the Lucas toolbox 2014 (13)] and plotted on log units. Additional scales with white fluorescent light and narrowband green (550 ± 23 nm) light are provided and aligned for equivalent biological potency [e.g., “10 lux” (1.5 μW/cm2) narrowband green LED light has the same biological potency as 4.7 lux while fluorescent light: 2.92 melanopic lux]. Fifty-five percent of the papers (39/71) did not report any spectral or color information about their light, so SPD for fluorescent white light was assumed for conversion. Overall density is represented by the black line. Individual papers are plotted as dots, and color coded for the reported unit type; 62/71 (87%) reported light intensities in lux alone. Both mode (tallest stack) and median intensities reported as “dim light” in our search were 10 lux fluorescent white light (6.2 melanopic lux) – bright enough for phase shifting (15), entrainment (16), or melatonin suppression (17) in humans. The three papers below 0.1 melanopic lux all reported the use of red dim light; all other papers used either white light or did not report color information. Range of intensities used in Gorman lab rodent studies is indicated by the green bracket, and extends to lower values outside the bounds of the figure (see Table 1). Search strategy: A pubmed search was performed on October 23, 2020 with search terms: “human circadian dim light.” Results were filtered for “2,000-present” and “Journal Article,” which resulted in a total of 509 articles, and sorted for “Best Match.” The first 100 results were downloaded and screened for light intensity information. In total, 29 papers were excluded because they were either review papers (n = 11), used non-human subjects (n = 13), or reported neither intensity nor source (n = 5). From each paper the brightest light referred to as “dim light” was recorded. If the paper reported a range (e.g., 1–5 lux, or <5 lux), the highest value within the range was used. If multiple units were reported (e.g., Lux and μW/cm²) irradiance was used over illuminance.