A controlled lab experiment on the effect of light exposure of different source sizes on subjective alertness and cognitive performance during the post-lunch dip

The spatial distribution of light within one’s field of view plays a moderating role in both visual and non-visual effects. Besides the main vector of light direction, another aspect of the luminous spatial distribution is the perceived size of the light source expressed in a solid angle. In this project, we investigated the effect of frontal light exposure with two different solid angles and luminance settings. The vertically measured illuminance was similar in both settings at ca. 510 lx, corresponding to a melanopic Equivalent Daylight Illuminance of 580 lx. In an early afternoon laboratory study in a within-subject design, 39 participants underwent a 2-h protocol consisting of cognitive performance tasks, and subjective assessments of sleepiness and visual comfort. The data showed mixed results, with the light exposure from the source of higher luminance and smaller solid angle leading to decreased subjective sleepiness, however, at the cost of decreased reaction speed in a cognitive task.