Smiling asleep: A study of happy emotional expressions during adult sleep.

Human foetuses and newborns smile first during sleep, before they smile while awake and interacting with caregivers. Whether smiling persists during adult sleep, and expresses inner joy, is yet unknown. Smiles were looked for during night-time video-polysomnography combined with electromyography of the zygomatic and orbicularis oculi muscles in 100 controls, 22 patients with sleepwalking and 52 patients with rapid eye movement (REM) sleep behaviour disorder.