Diurnal spectral sensitivity of the acute alerting effects of light.

STUDY OBJECTIVES Previous studies have demonstrated short-wavelength sensitivity for the acute alerting response to nocturnal light exposure. We assessed daytime spectral sensitivity in alertness, performance, and waking electroencephalogram (EEG). DESIGN Between-subjects (n = 8 per group). SETTING Inpatient intensive physiologic monitoring unit. PARTICIPANTS Sixteen healthy young adults (mean age ± standard deviation = 23.8 ± 2.7 y). INTERVENTIONS Equal photon density exposure (2.8 × 10(13) photons/cm(2)/s) to monochromatic 460 nm (blue) or 555 nm (green) light for 6.5 h centered in the middle of the 16-h episode of wakefulness during the biological day. Results were compared retrospectively to 16 individuals who were administered the same light exposure during the night. MEASUREMENTS AND RESULTS Daytime and nighttime 460-nm light exposure significantly improved auditory reaction time (P < 0.01 and P < 0.05, respectively) and reduced attentional lapses (P < 0.05), and improved EEG correlates of alertness compared to 555-nm exposure. Whereas subjective sleepiness ratings did not differ between the two spectral conditions during the daytime (P > 0.05), 460-nm light exposure at night significantly reduced subjective sleepiness compared to 555-nm light exposure at night (P < 0.05). Moreover, nighttime 460-nm exposure improved alertness to near-daytime levels. CONCLUSIONS The alerting effects of short-wavelength 460-nm light are mediated by counteracting both the circadian drive for sleepiness and homeostatic sleep pressure at night, but only via reducing the effects of homeostatic sleep pressure during the day.