Mind the Individual: Physiological Reactions to Long-Term Colored Light Exposure Assessed by SPA-fNIRS (Knisely Award Honorary Lecture).

Colored light exposure (CLE) is a common occurrence in everyday life. In today's modern society, we are increasingly exposed to various sources of colored light, including advertisements, computer and smartphone displays, energy-saving light bulbs, private settings, and street lighting with LEDs. Therefore, the effects of colored light on human physiology are becoming a subject of growing interest to both the research community and society. To gain a deeper understanding of the complex effects of long-term CLE on the human body, we have conducted a series of pioneering studies (with results published in 16 papers) over the past 8 years using the systemic physiology augmented functional near-infrared spectroscopy (SPA-fNIRS) approach. This approach facilitates concurrent brain activity measurement using fNIRS alongside multiple systemic physiological parameters. Our studies yielded a number of new findings: (i) We discovered the need for both group-level and individual-level analyses to fully understand the effects of CLE on human physiology. (ii) We found that CLE has relatively long-lasting effects on cerebrovascular and systemic physiology in humans. (iii) Yellow and short-wavelength light (i.e., blue and violet) were found to induce higher brain activation in the prefrontal cortex and visual cortex, respectively, compared to other colored lights (e.g., red, orange, and green). (iv) We observed that healthy adults exhibited a range of different types of physiological responses. Machine learning enabled us to classify the response patterns and provide new insights into the common response patterns between subjects. Given the rapid exposure of our society to new and increasing lighting levels, this research's findings are of interest and relevance to the scientific community, medical professionals, and society.