Non-Image-Forming Effects of Daytime Electric Light Exposure in Humans: A Systematic Review and Meta-Analyses of Physiological, Cognitive, and Subjective Outcomes

Light exerts numerous nonvisual or non-image-forming, effects – such as its impact on melatonin suppression, circadian phase-shifts, sleep quality, and alertness. While research typically focuses on evening and nighttime effects, a growing number of studies described the nonvisual impact of light during daytime (from 7am to 7pm). In this work, we systematically analyzed 141 peer-reviewed articles (150 studies) published by March 2023, encompassing 5401 healthy volunteers mainly aged 18–30. We calculated effect sizes using data extracted from the text, tables, or graphs and conducted 43 individual multilevel meta-analyses of physiological, cognitive, and subjective outcomes, divided into effects of light intensity and light spectral properties. We retained 17 meta-analyses that were based on data from more than 10 studies. Our results showed that higher light intensity enhances EEG brain wave-activity associated with heightened cognitive processing and attention, leads to faster response times, and reduces subjective sleepiness. The effect sizes were small. Meta-analyses of the effects of spectral characteristics of light did not show any significant summary effects. Overall, our findings suggest that higher ambient light intensities during daytime may be beneficial, but benefits are likely to be modest in healthy young human adults, who already are performing at near-optimal levels.