In the last eighteen years there has been the identification of a novel photopigment, melanopsin, and its subsequent localization to human intrinsically photosensitive retinal ganglion cells (ipRGCs). Since melanopsin’s peak sensitivity is in the short wavelength portion of the visible spectrum (from 447 nm to 484 nm), there has been a steady increase in studies investigating the physiological effects of blue light. This thesis examines polychromatic light mixtures of blue light for circadian, neuroendocrine and neurobehavioral effects in humans. White blue-enriched fluorescent lamps were tested at equal photon densities for increased efficacy for melatonin suppression, increased alertness, and circadian phase shifting. Results demonstrated that compared to white fluorescent light, blue-enriched fluorescent light was significantly stronger for suppressing melatonin and resulted in significantly reduced subjective sleepiness. Blue-enriched light, however, was not significantly stronger in eliciting circadian phase-delay or increasing objective measures of alertness. Next, blue-appearing narrowband solid-state light was examined for its ability to acutely suppress nocturnal melatonin as well as enhance cognitive performance and alertness in healthy men and women when compared to dim white lighting. The results demonstrated that narrowband blue solid-state light was significantly stronger for melatonin suppression compared to dim white light. Subjective and objective assessments of alertness, however, were not significantly increased by blue-enriched light exposure. The final study tested the hypothesis that certain combinations of light wavelengths are additive or opponent to the photoreceptor system that mediates the melatonin suppression. The results demonstrated that the melatonin suppression responses to dual narrow bandwidth light combinations were not significantly different from single wavelength exposures. Taken together, the results suggest that melanopsin sensitivity is not the sole consideration for predicting the efficacy of white polychromatic lighting. The different effects of blue light on alertness, circadian phase-shifting and melatonin suppression imply a either a context dependent sensitivity and/or differential involvement of the classical photoreceptors in these light responses.