Post-Nap Sleep Inertia in Narcolepsy Type 1 and Idiopathic Hypersomnia

Abstract Study objectives To compare post-nap sleep inertia between patients with idiopathic hypersomnia (IH) and narcolepsy type 1 (NT1) by assessing changes in subjective sleepiness and objective performance. Methods In 128 drug-free patients (50 IH, 78 NT1), standardized overnight polysomnography and Multiple Sleep Latency Testing were performed, with 5-minute Psychomotor Vigilance Task (PVT) and Karolinska Sleepiness Scale (KSS) assessments immediately before and after the 11:00 AM MSLT nap. Sleep inertia was defined as pre- to post-nap changes. Associations with clinical and polysomnographic variables were examined. Results NT1 and IH differed in nap duration and sleep architecture, with greater REM sleep in NT1 and longer N2 sleep in IH. Despite these differences, both groups exhibited comparable post-nap increases in KSS and in PVT lapses, with higher absolute KSS values in IH but no between-group differences in the magnitude of change. Post-nap KSS showed a modest correlation with post-nap PVT lapses, whereas no association was observed in the pre-nap condition. Post-nap subjective impairment was related to baseline KSS and IH severity scores, while post-nap performance impairment was primarily driven by pre-nap lapse count. No nap sleep-stage variables independently accounted for performance outcomes. MSLT-derived features showed weak and inconsistent associations with subjective outcomes and no association with performance in IH and NT1. Conclusions Sleep inertia immediately after a short morning nap is a reproducible phenomenon in IH and NT1, largely independent of nap sleep architecture. Post-nap impairment mainly reflects underlying sleepiness and performance deficits, supporting a shared mechanism of sleep inertia across central hypersomnolence disorders.