This doctoral project seeks to answer the question about the essence of functioning, disability and health in the lived experience of persons with any kind of primary sleep disorder. Its overall objective is the development of a first version of Core Sets of categories of the International Classification of Functioning, Disability and Health (ICF) in an evidence- and consensus-based process. To this end, four separate studies exploring different perspectives (researcher, clinical, patient, health professional) have been conducted and their results provided the evidence basis for selecting the relevant categories for the ICF Core Sets for Sleep Disorders during an international consensus conference. The doctoral thesis first-authored by the doctoral candidate therefore consists of five separate publications (1 Systematic Review, 2 Patient Studies, 1 Expert Survey, 1 Conference Results) that describe the different steps in the development process.