Study design: Prospective study concerning children with Down syndrome (age 1-18y). Parents will complete two sleep symptom questionnaires (PSQ-SRBD and CSHQ) and children will be evaluated by a paediatric pulmonologist and allergist with skin allergy test. An Ear, Nose and Throat specialist will complete two questionnaires (CAS-15 and SCR) carrying a nasopharyngoscopy. Then, children will perform an overnight polysomnography in the Department of Paediatric Functional Investigations of University Hospital in Nice.