High sensitivity and interindividual variability in the response of the human circadian system to evening light

Abstract

Before the invention of electric lighting, humans were primarily exposed to intense (>300 lux) or dim (<30 lux) environmental light—stimuli at extreme ends of the circadian system’s dose–response curve to light. Today, humans spend hours per day exposed to intermediate light intensities (30–300 lux), particularly in the evening. Interindividual differences in sensitivity to evening light in this intensity range could therefore represent a source of vulnerability to circadian disruption by modern lighting. We characterized individual-level dose–response curves to light-induced melatonin suppression using a within-subjects protocol. Fifty-five participants (aged 18–30) were exposed to a dim control (<1 lux) and a range of experimental light levels (10–2,000 lux for 5 h) in the evening. Melatonin suppression was determined for each light level, and the effective dose for 50% suppression (ED50) was computed at individual and group levels. The group-level fitted ED50 was 24.60 lux, indicating that the circadian system is highly sensitive to evening light at typical indoor levels. Light intensities of 10, 30, and 50 lux resulted in later apparent melatonin onsets by 22, 77, and 109 min, respectively. Individual-level ED50 values ranged by over an order of magnitude (6 lux in the most sensitive individual, 350 lux in the least sensitive individual), with a 26% coefficient of variation. These findings demonstrate that the same evening-light environment is registered by the circadian system very differently between individuals. This interindividual variability may be an important factor for determining the circadian clock’s role in human health and disease.

Accuracy of the GENEActiv Device for Measuring Light Exposure in Sleep and Circadian Research

“…These findings will help to improve interpretation of assessments using this sensor, which is critical in research studies with a circadian component. The human circadian system responds non-linearly to light intensity, with high sensitivity to light levels below 100 lux [2,3,19]. Consequently, it is important for devices used in sleep and circadian studies to accurately capture light intensities at these levels, which are typically experienced in everyday life.…”

“…Light is the key zeitgeber (time cue) responsible for synchronizing the circadian clock [1] and is therefore a critical measure in studies assessing circadian outcomes. The human circadian system is highly sensitive to light, with its dynamic range overlapping with typical indoor light levels [2,3], and peak sensitivity to blue light [4,5]. Accurate assessment of an individual's light exposure profile, including under typical indoor lighting, is therefore essential to understanding the impact of light on the circadian system.…”

Two-Community Noisy Kuramoto Model Suggests Mechanism for Splitting in the Suprachiasmatic Nucleus

“…Figure 3B and D show the situation in which the transition occurs in the opposite direction (from antialigned to aligned). Phillips et al (2019) recently showed that there are interindividual differences in light sensitivity between subjects, which may affect sleep and circadian timing. These interindividual differences in light sensitivity may also be the reason that not all animals will arrive in the split state when subjected to constant light or a forced desynchrony protocol.…”

Characterizing the Human Heart Rate Circadian Pacemaker through Widely Available Wearable Devices

“…Other circadian mechanisms exhibit similar variation. As a recent example, melatonin suppression due to light exposure was found to vary significantly between subjects (18).…”