The present study was performed according to the protocol approved by the Ethics Review Committee of Nagoya City University Graduate School of Medical Sciences and Nagoya City University Hospital (No. 60160164).
We studied eight subjects (mean age ± SD, 22 ± 3 year, seven males and a female) who had normal color vision, were not taking any medications for >2 weeks, and displayed a normal sinus rhythm on electrocardiogram (ECG) at rest. All subjects gave their written informed consent to participate in this study.
An organic electroluminescent (OEL) lighting device developed for research purposes was used. The device consisted of four OEL panels (VELVE OLED Lighting Module with adjustable RGB color and brightness, 55× 55 mm square, Mitsubishi Chemical Pioneer OLED Lighting Corporation, Tokyo, Japan) that were linearly aligned with two panels at the both sides inclining inward with an angle of 40°. Using a custom-made experimental frame, the lighting device was secured at a constant distance of 24 cm from the eyes of subject lying on a bed in the supine position so that the four OEL panels were aligned across their body axis and able to be moved in their sagittal plane keeping the light axes always facing their eyes (Figure 1).
Although the device was able to emit red, green, and blue in any combinations, only 100% blue light was used in this study. Front illuminance were 15.4 lx at subject’s eye position, chromaticity (x, y) was (0.14, 0.16), and the melanopsin-stimulating component that was estimated from the melanoptic spectral efficiency curve adjusted for the effect of human pre-receptoral filtering [17-19] were 75% of the total photon flux density (0.377 µmol/[m²·s] ).
Subjects were instructed not to consume food or beverages containing caffeine or alcohol after 21:00 the previous night. The experiments were performed between 10:00 and 17:00 in a calm, light-shielded, and air-conditioned (24 ± 2 °C) laboratory more than two hour after a light meal. Subjects were laid supine on a bed and instructed to continue to look at the mark on the ceiling just above their heads while they were exposed to light, and not to look directly the light source. After a dark adaptation for 10 min, the blue light from the device was exposed for 5 min from each of six angles (0º, 30º, 45º, 135º, 150º, and 180º). The order of the angle was counterbalanced among subjects. These was a 10-min dark interval period after light exposure from each angle.
Measurement and data analysis
During the experiment including light exposure and dark periods, ECG was recorded continuously with bipolar CM5 lead with a bioelectric amplifier (Biotop mini, East Medic Corporation, Kanazawa, Japan), digitized at 500 Hz with an analog-to-digital converter (AIO-163202FX-USB, CONTEC Corporation, Osaka, Japan), and stored in a hard disk.
From ECG signal, R-R interval time series were obtained and divided into segments of 5-min light exposure and 10-min dark periods for each incident angle. For each segment, mean heart rate, standard deviation of R-R interval (SDNN), and low-frequency (LF, 0.04-0.15 Hz) and the high-frequency (HF, 0.15-0.40 Hz) components of heart rate variability (HRV) were computed.
Statistical Analyses System version 9.4 (SAS institute Inc., Cary, NC, USA) was used for the statistical analysis. The Mixed procedure was used for analysis of variance for repeated measures with incident angle, light-to-dark, and exposure order as the fixed effects and subject as the random effect. P <0.05 was considered to be statistically significant and Bonferroni adjustment was used to keep type 1 error level in multiple comparisons.