Introduction: Negative effects of psychotherapy encompass unwanted events caused by the treatment. Evidence exists that cognitive behavioral therapy for insomnia (CBT-I) can lead to negative effects (e.g., exhaustion, reduced motivation). Psychometrically validated instruments are needed to enable systematic investigation of negative effects and their consequences. This study aimed to develop an insomnia-specific extension for the Negative Effect Questionnaire (NEQ) and to evaluate its psychometric properties.

Method: Based on a literature review, an item pool was developed as an extension to the NEQ. This item pool was iteratively refined. Both the NEQ and the developed item pool were administered to participants enrolled in a stepped-care model for insomnia that includes an internet-delivered CBT-I. The psychometric properties of the item pool were evaluated using confirmatory factor analyses (CFAs). Additionally, responses to open-ended questions were categorized and analyzed.

Results: Data from 210 participants were available. In the newly developed item pool, participants reported a mean of 4.7 negative effects (range: 0 to 22). CFA indicated a good fit (RSMEA=0.05; SRMR=0.07) of a model with five latent factors (somatic symptoms, cognitive symptoms, safety, emotional symptoms, daytime functioning). The correlation analyses between the model's latent factors and the NEQ subscales indicated adequate discriminant validity.

Discussion: The observed prevalence rates of insomnia-specific negative effects highlight the need to assess the negative effects of CBT-I with an insomnia-specific instrument. The CFA suggests that the 24-item NEQ-Insomnia is a valid instrument, suitable for standalone use or as an extension to the NEQ for assessing the negative effects of CBT-I.