Effect of illuminance and color temperature on lowering of physiological activity.

To investigate how illuminance and color temperature in illumination affect the autonomic nervous system and central nervous system in conditions tending to lower physiological activity, and with an ordinary residential setting in mind, we performed an experiment on 8 healthy male subjects. The experimental conditions consisted of 4 conditions provided by a combination of 2 levels of color temperature (3000 K, 5000 K) and 2 levels of illuminance (30 lx, 150 lx). Physiological measurement was carried out during a process of 22 minutes of light exposure followed by 20 minutes of sleep in darkness. Heart rate variability (HRV) was used as an index of the autonomic nervous system, and alpha attenuation coefficient (AAC) and mean frequency of EEG were used as indices of the central nervous system. Subjective evaluation of drowsiness during the experiment was also carried out immediately following the 20 minutes sleep. No effect on HRV from illumination was noted, but significantly (p < 0.05) lower values for AAC were obtained under 3000 K conditions than 5000 K conditions in measurements during the first half of light exposure (Session 1). During alpha attenuation testing, significantly (p < 0.05) lower values for mean frequency in the theta-beta EEG bandwidth were also obtained under 3000 K conditions than 5000 K conditions, but that pattern persisted in measurement during the second half of light exposure (Session 2). Subjective drowsiness was also higher under 3000 K conditions than 5000 K conditions. These results suggest that low color temperature light creates a smooth lowering of central nervous system activity, and that low color temperature illumination can be used effectively in a bedroom or other such environment where it is desirable to facilitate lowered physiological activity.