The lighting environment at indoor workplaces is important not only to provide vision and visual comfort, but also for light's direct influence on human physiology, cognitive performance and mood. The purpose of this ongoing study is to investigate the impact of a dynamic control of combined daylight and artificial lighting on office users' visual comfort as well as on alertness and cognitive performance. We are going to evaluate the impact of two different office lighting conditions in a quasi-real setting on subjective alertness ratings in healthy young participants over several days. We will compare an office with optimized daylighting and artificial lighting, operated by a new control system with a standard office room, where the lighting/shading can be changed manually. The aim of this balanced crossover within study design is to show that exposure to optimized dynamic lighting control over several days is superior on subjective alertness and glare indexes, when compared to a conventional lighting control. Here, we present some preliminary results from the first six participants on the comparison of subjective evaluations of alertness and the objective monitored (day-) light exposures and glare index Daylight Glare Probability (DGP) in the two different conditions over one week (five days) each.