The effects of different light intensities on temperature and alertness were investigated. It was hypothesized that temperature and alertness would be affected by certain light intensities but only during the melatonin release period (after 2100 h). Fifteen subjects were tested under three levels of light known to suppress melatonin (500, 1000, and 5000 lx) and a level known not to affect melatonin (50 lx). Subjects were tested on four occasions from 1700 until 2300 h. Tympanic temperature and measures of alertness (EEG power and frequency and self-reports) were obtained before and after melatonin onset. There were no differences in any measure prior to the melatonin onset, increases in temperature and alertness occurred only after melatonin onset. Temperatures and self-reported alertness scores obtained under light intensities of 500, 1000, and 5000 lx were elevated relative to those obtained under 50 lx but were not significantly different from each other. The results suggest that melatonin may be involved in mediating the effects of light on temperature and alertness and that 500 lx may be near the threshold for significant melatonin suppression, temperature enhancement, and increases in alertness.