There is previous evidence that using high correlated colour temperature (T_cp), white light can improve concentration and alertness, resulting in better productivity in office environments. In this study, it was investigated whether the same applies to students exposed to light, while attending lectures. The study consisted of two sequential field studies, conducted during spring and autumn. The subjects were exposed to light provided either by 4,000 K or 17,000 K T5 fluorescent lamps for 90 minute lecture periods. The subjects were asked to judge their level of alertness on a 9-step scale at the beginning, and then at the end of each period. In spring, no change in alertness during the lecture period was detected. There was neither shown correlation between the T_cp of light and subjective alertness. Instead, the study in autumn found that the decrease in alertness during lectures was significant, and post-lunch dip effect appeared strong in the afternoon. The autumn study indicated that exposure of the subjects to a 17,000 K light source in afternoon lectures, potentially assists students to maintain higher levels of alertness, as compared with the results of the 4,000 K light. This study therefore suggests that the colour temperature, and the timing of the light exposure, play important roles in student alertness in lecture room environments.