Living in Biological Darkness: Objective Sleepiness and the Pupillary Light Responses Are Affected by Different Metameric Lighting Conditions during Daytime

Abstract

Nighttime melatonin suppression is the most commonly used method to indirectly quantify acute nonvisual light effects. Since light is the principal zeitgeber in humans, there is a need to assess its strength during daytime as well. This is especially important since humans evolved under natural daylight but now often spend their time indoors under artificial light, resulting in a different quality and quantity of light. We tested whether the pupillary light response (PLR) could be used as a marker for nonvisual light effects during daytime. We also recorded the wake electroencephalogram to objectively determine changes in daytime sleepiness between different illuminance levels and/or spectral compositions of light. In total, 72 participants visited the laboratory 4 times for 3-h light exposures. All participants underwent a dim-light condition and either 3 metameric daytime light exposures with different spectral compositions of polychromatic white light (100 photopic lux, peak wavelengths at 435 nm or 480 nm, enriched with longer wavelengths of light) or 3 different illuminances (200, 600, and 1200 photopic lux) with 1 metameric lighting condition (peak wavelength at 435 nm or 480 nm; 24 participants each). The results show that the PLR was sensitive to both spectral differences between metameric lighting conditions and different illuminances in a dose-responsive manner, depending on melanopic irradiance. Objective sleepiness was significantly reduced, depending on melanopic irradiance, at low illuminance (100 lux) and showed fewer differences at higher illuminance. Since many people are exposed to such low illuminance for most of their day—living in biological darkness—our results imply that optimizing the light spectrum could be important to improve daytime alertness. Our results suggest the PLR as a noninvasive physiological marker for ambient light exposure effects during daytime. These findings may be applied to assess light-dependent zeitgeber strength and evaluate lighting improvements at workplaces, schools, hospitals, and homes.

Effects of Narrowband Light on Choroidal Thickness and the Pupil

“…This variation may be due to individual subjects state of arousal; studies have suggested that a correlation exists between the PIPR and subjective sleepiness, such that the PIPR is smaller when subjects are sleepier. 49 Although we did not measure sleepiness, it is possible that exposure to 1 hour of darkness increased subjects’ sleepiness. Because we did not observe a significant difference in the 6-second PIPR after 1 hour of dark exposure, our results suggest that 5 minutes of dark adaptation should be sufficient to obtain a stable PIPR response.…”

“…These findings are consistent with previous studies demonstrating the effects of light- and dark-adapted conditions on pupil constriction. 49 , 50 …”

The Role of Daylight for Humans: Gaps in Current Knowledge

“…The direction of light seems to be crucial at least for electric light that was shown to affect melatonin suppression [107][108][109] and sleep [110]. From laboratory studies with electric lighting interventions during the daytime, mixed dose-response effects have been reported for intensity, duration and spectral composition: Some studies found effects of daytime/evening electric light exposure on subjective and/or objective variables [52,86,89,115], while others did not [82,116]. Few studies have been conducted under daylight conditions only or mixed daylight/electric light conditions, and most of these are applied or semi-natural studies [40,65,78].…”

“…Many laboratory studies with steady state electric lighting showed light-dependent alertness[45,46,49,[81][82][83] and cognitive repercussions[46,53,55]. A few studies have looked at spectrally tuned electrical light conditions and alertness and cognition[76,[84][85][86].…”

Perspective: Daylight Saving Time—An Advocacy for a Balanced View and against Fanning Fear

“…In recent years however, the effects of artificial light, in addition to natural sunlight, have gained increasing interest, particularly with the advent of light-emitting diodes (LEDs; see e.g., Blume, Garbazza, and Spitschan [20]) whose light spectrum is "circadian active". Moreover, natural daylight plays a decreasing role in modern societies, with some authors even arguing that many people live in "biological darkness" [21]. In the DST/ST debate, this is a crunch point as it relates to the question of how much the body clock depends on the apex of the sun's motion coinciding with 12 pm/noon in a world with an increasingly complex light environment.…”