Light has two main effects on alertness. Firstly, light as a stimulant increases alertness and improves performance, depending on its wavelength and intensity. Blue-enriched light (specifically blue light around 480 nanometers, the peak of melanopsin sensitivity) and high intensity light directly alerts the brain and can be used at times when alertness and productivity need to be heightened; for example, during daytime or at work.
Light has a second effect on alertness via its impact on our 24-hour circadian clock. Alertness and performance rhythms are strongly regulated by the clock, so when the clock is shifted (e.g. staying up later or waking earlier), the timing of peak alertness and performance shifts accordingly. Maintaining exposure to a stable light-dark cycle (bright blue-enriched days and dark nights, and a habitual sleeping pattern) is needed to maintain good circadian entrainment, including alertness and performance patterns.