The aim of the study is to investigate effects of light on attention during the day. In particular, changes in spectral composition and prolonged exposure durations are analyzed. The study design was developed to explicitly refer to working days of fulltime daytime workers and therefore increase the transferability to real-life work situations. A combination of subjective scales and psychomotoric tasks is used to address different types of cognitive processes and get a holistic picture of the attentional state. In general, this study contributes to a more precise description of alerting effects of light that may lead to lighting designs that actively support employees at their workplace in the future.