Individual differences in light sensitivity affect sleep and circadian rhythms

Abstract

Artificial lighting is omnipresent in contemporary society with disruptive consequences for human sleep and circadian rhythms because of overexposure to light, particularly in the evening/night hours. Recent evidence shows large individual variations in circadian photosensitivity, such as melatonin suppression, due to artificial light exposure. Despite the emerging body of research indicating that the effects of light on sleep and circadian rhythms vary dramatically across individuals, recommendations for appropriate light exposure in real-life settings rarely consider such individual effects. This review addresses recently identified links among individual traits, for example, age, sex, chronotype, genetic haplotypes, and the effects of evening/night light on sleep and circadian hallmarks, based on human laboratory and field studies. Target biological mechanisms for individual differences in light sensitivity include differences occurring within the retina and downstream, such as the central circadian clock. This review also highlights that there are wide gaps of uncertainty, despite the growing awareness that individual differences shape the effects of evening/night light on sleep and circadian physiology. These include (1) why do certain individual traits differentially affect the influence of light on sleep and circadian rhythms; (2) what is the translational value of individual differences in light sensitivity in populations typically exposed to light at night, such as night shift workers; and (3) what is the magnitude of individual differences in light sensitivity in population-based studies? Collectively, the current findings provide strong support for considering individual differences when defining optimal lighting specifications, thus allowing for personalized lighting solutions that promote quality of life and health.

Keywords: behavioral interventions; circadian rhythms; individual traits; light exposure; sleep–wake regulation.

Figure 1.

Conceptual scheme of individual differences in light sensitivity effects on sleep and circadian rhythms. Photic input, through, for example, light bulbs and electronic devices, impinge onto the human SCN (central circadian clock) [12], resulting in a multitude of NIF responses to light (output) [2, 13]. Recent evidence shows individual differences in circadian photosensitivity [11]. High sensitivity leads to increased melatonin suppression by evening/night light exposure (red line), as compared to the group average (dashed black line). In contrast, low sensitivity leads to minimal (if any) melatonin suppression by evening/night light exposure (blue line), as compared to the group average (dashed black line). Although not fully established, similar individual differences in light sensitivity are expected to occur for sleep quality (here, indexed as e.g. slow-wave activity, 0.75–4.5 Hz). Accordingly, high sensitivity may result in less slow-wave activity, particularly at the beginning of the sleep episode, subsequent to evening/night light exposure (red line), as compared to the group average (dashed black line). In contrast, low sensitivity may result in minimal (if any) effects of evening/night light exposure on slow-wave activity (blue line), as compared to the group average (dashed black line). Such individual differences in light sensitivity may be ascribed to a constellation of traits, including age, sex, chronotype, and genetic haplotypes (e.g. [42, 55, 56, 68, 69, 79, 89, 90, 96]). Albeit race/ethnicity influences sleep/circadian rhythms, there are currently no studies on this trait as mediating the effects of light exposure. Individual traits may influence the effects of light on sleep and circadian rhythms at the level of the eye (input) and/or downstream (circadian clock and beyond). Dashed lines to input, circadian clock and to output indicate that potential mechanisms are known for some individual traits, but not all (e.g. [3, 52, 53]).

Figure 2.

Assessing individual photosensitivity for potential personalized lighting recommendations. Assessment of individual photosensitivity in humans has come from human laboratory and field studies, which use different outcome measures to index sleep and circadian rhythms. By identifying individuals with high or low sensitivity to light, a potential “light diet” tailored to individual needs might become feasible (see text for detailed discussion).