The German version of the Sleep Inertia Questionnaire in central disorders of hypersomnolence: a validation study

Background Sleep inertia, or sleep drunkenness, is a transitional state between sleep and wakefulness characterized by cognitive and physical impairment. It is particularly relevant in central disorders of hypersomnolence (CDH), but validated instruments to assess sleep inertia in German-speaking populations are lacking. This study translated and validated the German version of the Sleep Inertia Questionnaire (SIQ) in patients with CDH and healthy controls. Methods We included 109 participants from the international Swiss Primary Hypersomnolence and Narcolepsy Cohort Study (iSPHYNCS): idiopathic hypersomnia (IH, n = 40), narcolepsy type 1 (NT1, n = 21), other CDH/narcolepsy borderland (NBL, n = 34), and healthy controls (HC, n = 14). Psychometric evaluation included confirmatory factor analysis (CFA), internal consistency, test-retest reliability, construct validity, discriminative validity, and gender-based analyses. Results A modified four-factor second-order CFA model showed acceptable overall fit, supporting the factorial validity of the German SIQ while suggesting that its structure is not identical to the original English version. The SIQ demonstrated excellent internal consistency (Cronbach’s alpha = 0.96) and test-retest reliability (intraclass correlation = 0.94). SIQ scores correlated strongly with fatigue (r = 0.79) and depression (r = 0.67), and moderately with sleepiness (r = 0.47) and sleep quality (r = 0.38). SIQ scores separated CDH from HC (AUC = 0.886) and IH from NT1 (AUC = 0.721). Females reported higher SIQ scores, particularly within IH. Conclusion The German SIQ is a reliable and valid instrument for assessing sleep inertia severity in CDH. Its factorial structure, group-level discriminative validity, and potential gender differences warrant further investigation in larger cohorts.