Insomnia is defined by the DSM-5 (American Psychiatric Association, 2013, p. 362) as a "predominant complaint of dissatisfaction with sleep quantity or quality", especially difficulty initiating or maintaining sleep or earlymorning awakening; insomnia is widespread (prevalence of 5.7% in the German population; Schlack, Hapke, Maske, Busch, & Cohrs, 2013) and tends to persist for years (Morin et al., 2009). This disorder has serious effects on patient's lives; for instance, insomnia is predictive of subsequent depression (Baglioni et al., 2011) and is associated with suicide risk independent of depression (Drapeau & Nadorff, 2017; Etindele Sosso, 2017). Cognitive behavioural therapy is the method of choice for treating patients with chronic insomnia (American