Short-term memory measurement is an important method to determine study fatigue as it pertains to learning outcome. This project examines the impact of classroom-lighting on circadian rhythm. A learning-recognition model using two-character Chinese word cards formed the basis of this study. Utilizing the discriminating ability in Signal Detection Theory as an index, students’ brain fag (mental exhaustion and fatigue), short-term memory and recognition ability are tested and analyzed. The subjects are placed under T5 fluorescent lamps of 3 typical colour temperatures and with 4 luminance levels. Within a certain length of time, students’ brain fag and recognition ability of short-term memory differs under different spectrum conditions and luminance levels. Low colour temperature and low luminance has a wake-up effect that improves short-term memory. Medium colour temperature is the optimum that can keep brain function appropriately excited. Under high luminance level, brain fag is more serious and learning efficiency is lower. Brain fatigue is most pronounced under high colour temperature. Obviously longer exposure time to the Chinese characters will result in higher recognition ability. This study has important implications in how to improve learning through properly designed lighting conditions.