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.