Literature has occasionally reported acute effects of office illuminance on cognitive performance during daytime. The current study was conducted to systematically investigate whether the type of task and level of task difficulty moderate the effect of daytime illuminance on cognitive functioning. Thirty healthy participants were assigned to high (∼1036 lux at eye level; melanopic EDI = 904 lux) vs. low (∼108 lux at eye level, melanopic EDI = 87 lux) illuminance (at 6500 K) during working hours, in which participants were tested on both easy and difficult versions of tasks probing sustained attention, response inhibition, conflict monitoring and working memory. Subjective sleepiness and mood were also measured. Results revealed that exposure to high vs. low illuminance significantly improved speed on the response inhibition task, and accuracy and speed on the working memory tasks. Moreover, when effects arose, these were moderated by task difficulty, consistently showing more pronounced effects for easy than for difficult trials. Notably, subjective sleepiness and negative mood remained unaffected by illuminance, and no statistically significant effects emerged for sustained attention and conflict monitoring. This study demonstrates that the diurnal cognitive effects of illuminance may be moderated by both types of cognitive task and task difficulty.