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Background: Sleep disturbances in schizophrenia are common throughout its course including in the prodrome, and have been mainly attributed to severity of symptoms and antipsychotic use. We aimed to investigate whether early course patients with schizophrenia and young non‐psychotic siblings of patients with schizophrenia also show sleep disturbances and whether sleep correlates with symptoms and functioning. Methods: Three study groups, that is, adults newly diagnosed with schizophrenia (n = 54), young non‐psychotic siblings of schizophrenia patients (n = 56) and a sample of healthy controls matched to the patients and siblings (n = 61) were evaluated on Horne and Ostberg Morningness‐Eveningness Questionnaire, Epworth Sleepiness Scale and Pittsburgh Sleep Quality Index. Severity of symptoms and functioning are assessed using the Positive and Negative Syndrome Scale and the Global Assessment of Functioning Scale, respectively. Age, gender, occupation and marital status were regarded as covariates, and differences between the three groups were evaluated using analysis of covariance. Results: Early course schizophrenia patients and non‐psychotic siblings of schizophrenia patients showed significantly reduced sleep quality relative to healthy controls (P <.001). Schizophrenia patients had significantly higher daytime sleepiness compared to controls (P <.001). Chronotypes in schizophrenia patients and unaffected siblings did not significantly differ from those of the healthy controls. Conclusions: Like chronic medicated schizophrenia patients, early course schizophrenia patients and young non‐psychotic siblings of individuals with schizophrenia have sleep disturbances. These findings indicate that sleep markers can distinguish unaffected siblings of schizophrenia from healthy controls and serve as an endophenotype for schizophrenia. [ABSTRACT FROM AUTHOR]