Spanish Translation and Psychometric Evidence of the Nightmare Disorder Index (NDI) in Adults from the General Population

Background: Nightmares are a type of sleep disorder characterised by vivid and distressing dreams that cause abrupt awakenings, leading to significant discomfort. In adults, recurrent nightmares can negatively impact quality of life, daytime functioning, and overall mental health. In this context, it is essential to have valid, reliable, and culturally appropriate psychological instruments that allow for an accurate assessment of this phenomenon. The aim of the present study was to translate and validate the Nightmare Disorder Index (NDI) into Spanish for use with Peruvian adults. Methods: A total of 507 adults (66.7% women) participated by completing a sociodemographic questionnaire and psychological instruments. Statistical analyses were conducted using structural equation modelling and item response theory. Results: The NDI demonstrated adequate content validity (V > 0.70), a unidimensional structure (CFI = 0.99, RMSEA = 0.06 [90% CI: 0.030, 0.102], SRMR = 0.03), and reliability (ω = 0.84, H = 0.94, r_xx = 0.79). In addition, invariance was observed across sex, and significant associations were found with depressive symptoms, generalised anxiety, and suicidal ideation. Item 3 showed the highest discrimination and information, and the scale proved to be accurate at higher levels of nightmare severity. Conclusions: The NDI presents adequate psychometric properties for the inference and interpretation of scores in the assessment of nightmares. Its use is recommended in both professional practice and research with the adult general population.