To assess the test-retest reliability of the polysomnography-multiple sleep latency test (PSG-MSLT) diagnostic classification and measures and to study the determinants of its variability in patients with narcolepsy type 1 (NT1) or with noncataplectic central disorders of hypersomnolence (NCHS): type 2 (NT2), idiopathic hypersomnia (IH), and unspecified hypersomnolence (unspecified excessive daytime sleepiness [UnsEDS]).