Human foetuses and newborns smile first during sleep, before they smile while awake and interacting with caregivers; adults may have true smiling and laughing, a true inner mirth, during sleep

Smiling asleep: A study of happy emotional expressions during adult sleep. Marion Clé et al. Journal of Sleep Research, https://doi.org/10.1111/jsr.12814

Abstract: 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. Autonomous reactions (heart rate and level of vasoconstriction) and the presence of rapid eye movements were examined during smiles and laughs. On visual examination of the face video clips synchronous with zygomatic contraction, 8% of controls smiled while asleep (7% in REM sleep and 1% in non‐REM sleep). Some patients with sleepwalking also smiled and laughed during N2 sleep and N3 parasomnia. Half of the patients with REM sleep behaviour disorder smiled and one‐third laughed, mostly during REM sleep. The 173 happy faces included mild smiles (24.8%), open‐mouth smiles (29.5%) and laughs (45.7%). More than half of the smiles were the Duchenne (genuine) type, including an active closure of the eyelids. Approximately half of the smiles and laughs were temporally associated with rapid eye movements. There was no increased heart rate variability during smiles and laughs. Two scenic behaviours including smiles and laughs suggested that the happy facial expression was associated with a happy dreaming scenario. Smiling and laughing occasionally persist during adult sleep. There are several lines of evidence suggesting that these happy emotional expressions reflect a true inner mirth.