Inclusion Criteria:  Patients with PWS genetically confirmed at least 16y old willing to participate and being able to be still in the MRI.  Exclusion Criteria:  Age < 16 years old Presence of uncontrolled Obstructive Sleep Apnea (Residual AHI ≥ 30). Narcolepsy or other sleep disorders with daily somnolence Intolerance to the heavy blanket in spite of previous adaptation sessions Active psychopathological disorder that may interfere with sleep Use of hypnotic or sleep regulator drugs (change of dose in the last 3 months) Contraindications for MRI Impossibility to stay still during MRI adquisition