Alerting effects of short-wavelength (blue) and long-wavelength (red) lights in the afternoon

• Light can be used to increase alertness in the afternoon, close to the post-lunch dip hours.
• Red light is a stronger alerting stimulus in the afternoon than blue light.
• Long-wavelength cones mediate the alerting effects of red light during the daytime.

Light has an acute effect on neuroendocrine responses, performance, and alertness. Most studies to date have linked the alerting effects of light to its ability to suppress melatonin, which is maximally sensitive to short-wavelength light. Recent studies, however, have shown alerting effects of white or narrowband short-wavelength lights during daytime, when melatonin levels are low. While the use of light at night to promote alertness is well understood, it is important to develop an understanding of how light impacts alertness during the daytime, especially during the post-lunch hours. The aim of the current study was to investigate how 48-minute exposures to short-wavelength (blue) light (40 lux, 18.9 microWatts/cm2 λmax = 470 nanometers [nm]) or long-wavelength (red) light (40 lux, 18.9 microWatts/cm2 λmax = 630 nm) close to the post-lunch dip hours affect electroencephalogram measures in participants with regular sleep schedules. Power in the alpha, alpha theta, and theta ranges was significantly lower (p < 0.05) after participants were exposed to red light than after they remained in darkness. Exposure to blue light reduced alpha and alpha theta power compared to darkness, but these differences did not reach statistical significance (p > 0.05). The present results extend those performed during the nighttime, and demonstrate that light can be used to increase alertness in the afternoon, close to the post-lunch dip hours. These results also suggest that acute melatonin suppression is not needed to elicit an alerting effect in humans.