This paper presents an investigation of the mediating roles of mood and perception in subjects' cognitive performance in a daylit workplace. We hypothesized that various daylighting conditions would be affective sources that convey emotional meanings to room occupants and thus affect their behaviors. We carried out a series of controlled experiments in a sunlit work setting. Between-subject comparisons among ten groups, seated at different spots in a sunlit room, suggested that both lighting and non-lighting factors simultaneously influence subjects' mood, which was correlated to their performance. Moderate amounts of sunlight and view, together with a sense of control and privacy, positively affected subjects' mood, which was negatively affected by the increased difficulty of assigned cognitive tasks.