Background: Williams syndrome (WS) is a neurodevelopmental genetic disorder characterised by physical abnormalities and a distinctive cognitive profile with intellectual disabilities (IDs) and learning difficulties. Methods: In our study, nine adolescents and young adults with WS and 9 age- and sex-matched typically developing (TD) participants underwent polysomnography. We examined sleep architecture, leg movements and the electroencephalogram (EEG) spectra of specific frequency bands at different scalp locations. Results: We found an atypical, WS characteristic sleep pattern with decreased sleep time, decreased sleep efficiency, increased wake time after sleep onset, increased non-rapid eye movement percentage, increased slow wave sleep, decreased rapid eye movement sleep percentage, increased number of leg movements and irregular sleep cycles. Patients with WS showed an increased delta and slow wave activity and decreased alpha and sigma activity in the spectral analysis of the EEG. Conclusions: Sleep maintenance and organisation are significantly affected in WS, while EEG spectra suggest increases in sleep pressure.