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

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.

A video polysomnographic study of spontaneous smiling during sleep in newborns

“…This association between REMs and smiles was recently mentioned during REM sleep in adults (Cle et al, 2019). Cecchini et al (2013) have shown, on the contrary, that the number of grouped REMs during AS was negatively correlated to the number of smiles, but positively correlated to the number of cries that occurred during the previous stimulated period of wakefulness.…”

“…Smiling is an elaborate and complex facial expression, the duration and pattern of which were shown to be similar to that of the adult social smile in our initial study (Challamel et al, 1985). Smiles persist during sleep in adults and also occur in REM sleep: a recent study showed that the majority of these smiles were Grade 2-3, lasted from 0.5 to 6 s, were more frequent in REM than in non-REM sleep, and that 70.5% of them occurred concomitantly to REMs (Cle et al, 2019). This study also showed that half of patients with REM sleep behaviour disorder (RBD) smiled, although that only some controls did.…”

“…Neonates display abundant muscle twitches during REM sleep that are generated by the brainstem (Blumberg & Plumeau, 2016). In the study on smiling in adults, Cle et al, 2019 found that a short smile (<1 s) corresponded to short, asymmetrical lip corner rising. These twitching events did not have the same characteristics as Grade 2 or 3 smiles: they were no more frequent in patients with RBD compared to controls, whereas patients with RBD had significantly more Grade 2 or 3 smiles compared to controls, suggesting that twitches and Grade 2 and 3 smiles may not be generated by the same structures.…”

Duchenne Smiles as Honest Signals of Chronic Positive Mood

“…Further afield, Gonzaga, Keltner, Londahl, and Smith (2001) showed that DSs correlate with self-report and partner-report estimates of love within romantic relationships. DSs even penetrate sleep; Clé et al (2019) showed that DSs during dreaming are associated with happy dreaming scenarios, reflecting a "true inner mirth." Taken together, these findings support the hypothesis that DSs arise in the presence of felt positive emotions.…”

Sleep, insomnia and neurological and mental disorders

“…Clé et al. () from Paris are probably among the first authors not only to write about smiling while being asleep, but also perform an empirical study on this phenomenon. In this study 100 control subjects, 22 patients with sleepwalking and 52 patients with REM sleep behaviour disorder were studied.…”