Mental health disorders are the leading cause of disability and illness in adolescents worldwide. Critical to progress in the prevention and treatment of mental health disorders is (1) understanding the neurobiological mechanism associated with these disorders and (2) identifying biomarkers that may predict disease vulnerability, treatment response and illness course. The sleep EEG is a powerful index of brain structure and function, providing a window onto the developing brain. Furthermore, during sleep the brain generates internal oscillatory activity reflecting subcortical and cortical network activity quantifiable using the EEG. This proposal combines sleep EEG measurements with state-of-the-art deep phenotyping of mental health in a transdiagnostic and dimensional manner in order to tackle this question. This project is funded by the Swiss National Science Foundation.