Cornelia de Lange syndrome (CdLS) is a rare genetic disorder characterized by growth retardation, intellectual disability, limb defects, typical facial dysmorphism, and other systemic involvement. Sleep disturbances have been frequently reported in CdLS, but these have not been completely characterized, and prevalence data are conflicting. The aim of this paper is to characterize and determine the prevalence of sleep disorders in CdLS patients by means of a validated questionnaire. From November 2012 to November 2013, we asked 46 consecutive parents/caregivers of CdLS patients aged more than 3 years old to fill out the sleep disturbances scale for children (SDSC). The subjects were also characterized by the presence of epilepsy, intellectual disability (ID), behavioral problems, CdLS severity score, gastroesophageal reflux disease (GERD), and genetic test results. An abnormal total sleep score was found in 7 patients (15.2%), 26 (56.5%) showed a borderline total score, and 18 (39.1%) had an abnormal score for at least one SDSC factor. In our study sleep disorders were found to be positively associated to presence of epilepsy, GERD, ID, and behavioral disturbances. No correlation was evident with specific mutations of the different genes, BMI, and severity score. Our results confirm that sleep disorders represent a common problem in CdLS, with higher incidence than in the normal population. In these patients sleep disorders seem to be more prevalent in comorbid settings, representing a clinical indicator for different medical and neuropsychiatric disorders. Better knowledge and characterization of typology of sleep disorders in CdLS patients could permit a more specific therapeutic approach.