Comparative Effects of Red and Blue LED Light on Melatonin Levels During Three-Hour Exposure in Healthy Adults

Abstract

Circadian rhythms, essential for regulating human physiology and behavior, are influenced by light exposure, particularly at night. This study examined the impact of red (631 nm) and blue (464 nm) LED light on melatonin secretion, a key circadian marker. Twelve participants aged 19-55 years were exposed to red and blue light for three hours (9:00 p.m.-midnight), with hourly saliva samples analyzed via ELISA to track melatonin levels. Initially, melatonin levels were comparable under both light conditions. After one hour, both lights suppressed melatonin, but differences emerged after two hours: blue light-maintained suppression, with levels at 7.5 pg/mL, while red light allowed recovery to 26.0 pg/mL (p = 0.019). This pattern persisted at the third hour. Blue light had stronger suppression effects, particularly in younger participants and men. These results underscore blue light's disruptive effects on circadian health and highlight red light as a less disruptive alternative for nighttime environments.

Keywords: LED light; blue light; circadian rhythms; integrative lighting; melatonin; non-visual effects; red light; salivary biomarkers.

Figure 1

Spectral power distribution (A) and CIE 1931 chromaticity diagram (B) for the two LED light sources used in this study. (A) The blue LED shows a peak in the short-wavelength region (464 nm), while the red LED peaks in the long-wavelength region (631 nm). (B) Chromaticity coordinates plotted on the CIE diagram indicate the distinct color characteristics of each LED, corresponding to their spectral peaks.

Figure 2

(A) Luminaires used in the experiments: at the top, the custom-built luminaires, and at the bottom, the luminaires without the diffuser, showing the LEDs. (B) Luminaire with red LEDs, calibrating the distance for the experiment. (C) Luminaire with blue LEDs, adjusted using a meter to ensure the correct distance.

Figure 3

(A) Calculation of α-opic weighted spectra for red and blue LED test sources from CIE S 026 Toolbox (v1.49a—November 2020) [22]. The left figures show the spectral power distributions (SPDs) of the red LED top (illuminance = 4598.3 lx) and the blue LED bottom (illuminance = 11837.8 lx). The middle panels illustrate the α-opic action spectra for S-cones, M-cones, L-cones, rhodopic, and melanopic sensitivities. The right figures display the resulting α-opic weighted spectra for the red and blue LED test sources, obtained by multiplying the SPDs with the respective α-opic action spectra. (B) Calculation of the relative spectral contribution to the circadian response for cool (green) and warm (purple) lights, as well as blue and red LEDs, respectively, according to Rea et al. [26,27].

Figure 4

Mean (± standard deviation) melatonin concentration (pg/mL) across the four time points (baseline, 1 h, 2 h, and 3 h) under blue and red light conditions for different participant groups: (A) all participants (n = 12), (B) older female participants (n = 4, ages 32–55), including a 7-h measurement at 3 a.m., (C) young participants (n = 8), (D) all female participants (n = 7), and (E) all male participants (n = 5). The X-axis represents the time points of saliva sample collection, and the Y-axis shows the mean melatonin concentration for each lighting condition. Trend lines with equations illustrate melatonin secretion over time for both lighting conditions. Significant differences (p < 0.05) between blue and red light conditions at the same specific time points are marked with an asterisk (*). Statistical analysis was performed using the Wilcoxon signed-rank test for related samples.