Light as a Modulator of Non-Image-Forming Brain Functions—Positive and Negative Impacts of Increasing Light Availability

Abstract

Light use is rising steeply, mainly because of the advent of light-emitting diode (LED) devices. LEDs are frequently blue-enriched light sources and may have different impacts on the non-image forming (NIF) system, which is maximally sensitive to blue-wavelength light. Most importantly, the timing of LED device use is widespread, leading to novel light exposure patterns on the NIF system. The goal of this narrative review is to discuss the multiple aspects that we think should be accounted for when attempting to predict how this situation will affect the NIF impact of light on brain functions. We first cover both the image-forming and NIF pathways of the brain. We then detail our current understanding of the impact of light on human cognition, sleep, alertness, and mood. Finally, we discuss questions concerning the adoption of LED lighting and screens, which offer new opportunities to improve well-being, but also raise concerns about increasing light exposure, which may be detrimental to health, particularly in the evening.

Regional response to light illuminance across the human hypothalamus

“…Animal research has established that the biological impact of light illuminance impinges on many subcortical structures, many of which regulate sleep and wakefulness. [1][2][3][4] How these findings translate to human beings is not established. Here, we took advantage of the relatively high resolution and signal-to-noise ratio of UHF 7T fMRI to determine how illuminance affects the activity of the hypothalamus as, based on animal research, it receives the densest projections from ipRGCs.…”

“…These analyses could for instance address whether the regional changes in activity of the hypothalamus we find are upstream of the repeatedly reported impact of light illuminance on the activity of the pulvinar in the thalamus. 1,24 Although it does not receive direct dense input from ipRGCs, it is likely to indirectly mediate the biological impact of light on ongoing cognitive activity. 29 These knowledge gaps are important to address because acting on light stands as a promising means to reduce high sleepiness and improve cognitive deficits during wakefulness as well as to facilitate sleep in the few hours preceding bedtime.…”

“…Light exerts multiple non-image-forming (NIF) biological effects that influence the quality of sleep and wakefulness, and higher illuminance is known to stimulate alertness and cognition. 1 The biological effects of light primarily rely on a subclass of retinal ganglion cells that are intrinsically photosensitive (ipRGCs) because they express the photopigment melanopsin, which is maximally sensitive to photons with wavelength ~480nm. IpRGCs combine the light signalling of rods and cones to their intrinsic photosensitivity and, collectively, the biological effects of light present a maximal sensitivity to the shorter blue wavelength of visible light.…”

“…2,3 The brain circuitry underlying the biological effects of light has mostly been uncovered in nocturnal rodent models. 1,2 Translation to diurnal human beings, where the later maturation of the cortex allows for complex cognitive processing, 5 remains scarce.…”

Exposure to light supresses the activity of the medial and superior amygdala during emotional processing

“…Previous research reported that the mechanisms of action of light on human mood could involve the production of serotonin and dopamine (3–5), the normalisation of rods and cones functioning in the retina (6), the improvement of circadian entrainment (3,6), or an impact on the brain emotional processing (7,8). Light’s influence on mood in humans may be mediated through a light-sensitive pathway of the retina (9,10) that detects environmental irradiance and is often referred to as the non-image forming (NIF) pathway (11). The NIF system mostly mediates light influence on functions not directly related to image formation, including melatonin suppression (12), stimulation of alertness and cognitive performance (13) and circadian entrainment (14).…”