Sleep as an Immune Biomarker: Disease-specific Phenotypes and the Principle of Time is Brain, Time is Sleep

secondary sleep abnormalities [e.g., multiple sclerosis (MS)], and highlight research gaps requiring future mechanistic, polysomnographic (PSG), and biomarker-driven investigations.Importantly, better recognition of sleep disturbances in AINDs offers a valuable diagnostic opportunity, as sleep symptoms may serve as early warning signs of encephalitis, signal relapse, or persist as residual disability affecting recovery trajectories.Understanding these manifestations can also guide clinicians toward targeted immunotherapy, sleep-focused interventions, and optimized long-term care. IntroductIonSleep is a vital biological process essential for metabolic regulation, cognitive functioning, and neurological health.Disturbances in sleep architecture are well recognized across Parkinson's disease, Alzheimer's disease, and Fatal familial insomnia, where they significantly affect quality of life. 1 Lately, a growing body of evidence has revealed that sleep dysfunction is not only common but often prominent in autoimmune neurological diseases (AINDs). 2 Autoimmune encephalitides and related syndromes driven by immunoglobulin G (IgG) antibodies targeting neuronal surface or synaptic antigens frequently manifest as complex sleep disturbances spanning hypersomnolence, insomnia, parasomnias, rapid eye movement (REM) sleep behavior disorder (RBD), and sleep-related breathing abnormalities. 3 Given the heterogeneity of immune-mediated sleep disorders, the rapid discovery of novel neuronal antibodies, and the absence of a consolidated evidence landscape, a literature review is urgently needed.This review attempts to systematically map the existing literature, identify key knowledge domains, clarify conceptual boundaries between immunologically driven sleep disorders (e.g., narcolepsy, anti-IgLON5 disease) and neuroimmunological diseases with