Relationship between Indoor Daytime Light Exposure and Circadian Phase Response under Laboratory Free-Living Conditions

In this study, we measured the physical quantity of light (irradiance) in several wavelength bands at eye and wrist level for 8 h under laboratory conditions simulating daily life. Twenty-two healthy male participants (mean age ± SD, 29.95 ± 8.13 years) were able to move freely, and eye-level data were compared with irradiance data obtained at the wrist. We estimated circadian phase shift from the differences of DLMO before and after light exposure while considering natural drift. In addition, we investigated the influence of activities and body positions on light exposure and validated the effect of irradiance on circadian phase response. We found lower average irradiance at eye level compared with wrist level in all three RGB bands. The blue light irradiance measured at eye level was significantly correlated with circadian phase shift, possibly because the irradiance at eye level more accurately reflected its effect on the biological clock than did the measurement at the wrist. It follows that quantitative measurements of wavelength spectra at eye level are necessary to assess the effect of ambient light on the human biological clock.