The optimal design and construction of high-performance buildings to ensure the inhabitants' health, both physically and mentally, has attracted many designers. Maintaining appropriate daylight is one of the most important variables in designing of high-quality residential buildings. Indeed, it has a great impact on creating a sense of desirability, health and relaxation. The main question of this study was the relationship between the intensity of light and the activity of areas outside the brain's default mode network and its effects on the level of sustained attention. Based on the research question, the hypothesis was presented as follows: Low light inhibits sustained attention and regions outside the DMN are expected to be activated by increased light. The study results indicated that the measurable intensity of daylight, can meaningfully influence the examinees. In other words, ambient light at the intensity of 197 lux may activate some brain areas with a defining role in cognitive processes. This would help in activation of the default mode network and helping one to stay even more vigilant during the whole process. This could help better designing architectural spaces such as residential buildings, classrooms and meeting rooms, in which light is a key content of design where vigilance is concerned. It is suggested that researchers, in future studies, pay attention to the effect of other factors on the intensity of illumination, such as Dimensions and type of architectural space, type of materials, color, and absorption coefficient of walls and floors. Manuscript profile