Study Objective: The aim of this study is to evaluate whether the cortical arousal level in subjects with primary insomnia varies according to their quality of nighttime sleep. Design: The P300 ERP was recorded each day, before and after sleep actigraphically monitored for 1 week. For each participant, the nights of worst- and best-quality sleep were identified. Participants: Seven subjects with primary insomnia and 7 control subjects. Measurements and Results: The P300 amplitudes were compared by considering group (insomnia vs control), night (worst vs best), and moment of recording (before vs after sleep) as factors. The subjects with insomnia showed higher P300 amplitudes, as compared with amplitudes of control subjects, only for the worst night of sleep. Conclusions: Results indicate a relationship between variations in sleep quality and cortical arousal level in subjects with primary insomnia.