Alerting effects of light are sensitive to very short wavelengths

In humans a range of non-image-forming (NIF) light responses (melatonin suppression, phase shifting and alertness) are short wavelength sensitive (440-480 nm). The aim of the current study was to assess the acute effect of three different short wavelength light pulses (420, 440 and 470 nm) and 600 nm light on subjective alertness. Healthy male subjects (n = 12, aged 27 +/- 4 years, mean +/- S.D.) were studied in 39, 4-day laboratory study sessions. The subjects were maintained in dim light (<8 lx) and on day 3 they were exposed to a single 4-h light pulse (07:15-11:15 h). Four monochromatic wavelengths were administered at two photon densities: 420 and 440 nm at 2.3 x 10(13)photons/cm(2)/s and 440, 470 and 600 nm at 6.2 x 10(13)photons/cm(2)/s. Subjective mood and alertness were assessed at 30 min intervals during the light exposure, using four 9-point VAS scales. Mixed model regression analysis was used to compare alertness and mood ratings during the 470 nm light to those recorded with the other four light conditions. There was a significant effect of duration of light exposure (p < 0.001) on alertness but no significant effect of subject. Compared to 470 nm light, alertness levels were significantly higher in 420 nm light and significantly lower in the 600 nm light (p < 0.05). These data (420 nm>470 nm>600 nm) suggest that subjective alertness may be maximally sensitive to very short wavelength light.