Cognitive behavior therapy for insomnia (CBT-I) is the first-line treatment for patients with insomnia disorder, including patients with severe mental disorders and comorbid insomnia. However, CBT-I is not sufficiently implemented in acute psychiatry settings. To make this treatment more accessible, we are currently adapting CBT-I to the needs of patients with severe psychiatric disorders in the form of a treatment program entitled SLEEPexpert. A core element of SLEEPexpert is keeping a sleep diary and restricting time in bed to increase sleep pressure. Here, we present a mobile application which supports the implementation of SLEEPexpert. The app is kept very simple, specifically designed for the target user group, and offers four main functionalities: entering information into the sleep diary, calculating the sleep efficiency and adapting the sleep window, delivering information on sleep and sleep disorders and accessing the recorded data in the sleep diary. Currently, we are preparing a usability test for the app aiming at fixing usability issues before running a clinical trial to assess the efficacy of this mHealth intervention.