Short-Wavelength Sensitivity for the Direct Effects of Light on Alertness, Vigilance, and the Waking Electroencephalogram in Humans

To assess the wavelength-dependent sensitivity of the acute effects of ocular light exposure on alertness, performance, waking electroencephalogram (EEG), and cortisol.A between-subjects design was employed to compare the effects of exposure to 460-nm or 555-nm light for 6.5 hours during the biological night.Intensive Physiological Monitoring Unit, Brigham and Women's Hospital, Boston, MA.Sixteen healthy adults (8 women; mean age +/- SD = 23.3 +/- 2.4 years).Subjects were exposed to equal photon densities (2.8 x 10(13) photons x cm(-2) x s(-1)) of either 460-nm (n = 8) or 555-nm (n = 8) monochromatic light for 6.5 hours, 15 minutes after mydriasis.Subjects underwent continuous EEG/electrooculogram recordings and completed a performance battery every 30 to 60 minutes. As compared with those exposed to 555-nm light, subjects exposed to 460-nm light had significantly lower subjective sleepiness ratings, decreased auditory reaction time, fewer attentional failures, decreased EEG power density in the delta-theta range (0.5-5.5 Hz), and increased EEG power density in the high-alpha range (9.5-10.5 Hz). Light had no direct effect on cortisol.Short-wavelength sensitivity to the acute alerting effects of light indicates that the visual photopic system is not the primary photoreceptor system mediating these responses to light. The frequency-specific changes in the waking EEG indicate that short-wavelength light is a powerful agent that immediately attenuates the negative effects of both homeostatic sleep pressure and the circadian drive for sleep on alertness, performance, and the ability to sustain attention.