SUMMARY Facial muscle contractions (FMC) are a commonly observed feature during sleep stages in human subjects. Previous studies have associated FMC to emotional expression during waking. Phasic features of rapid eye movement (REM) sleep, such as REMs have also been linked to an increase in limbic structure activity with a subsequent relation to emotional dream content. We hypothesized that FMC would be more frequent during REM sleep, and that FMC would correlate with the characteristic REMs of this sleep stage. The present study was designed to evaluate the density per minute of the phasic and sustained FMC of five facial muscles (frontalis, corrugator, orbicularis oculi, zygomatic major left and right) during all sleep stages, and to explore their relation with REMs density in six healthy participants during an 8 h sleep recording. Results showed a significant increase in FMC of all recorded muscles during REM sleep, both in frequency and duration. Additionally, as expected, during REM sleep there was a positive correlation among the facial muscles and between these and REMs. Nevertheless, although associated in number, both features (FMC and REMs) were never simultaneous. Our results suggest that limbic activation during REM sleep may be responsible for the enhancement of facial muscle activity, which may be consistent with the theoretical perspective of a higher emotional activity during REM sleep.k e y w o r d s EMG activity, facial muscles, rapid eye movements, REM sleep

Although motor activity is actively inhibited during rapid eye movement (REM) sleep, specific activations of the facial mimetic musculature have been observed during this stage, which may be associated with greater emotional dream mentation. Nevertheless, no specific biomarker of emotional valence or arousal related to dream content has been identified to date. In order to explore the electromyographic (EMG) activity (voltage, number, density and duration) of the corrugator and zygomaticus major muscles during REM sleep and its association with emotional dream mentation, this study performed a series of experimental awakenings after observing EMG facial activations during REM sleep. The study was performed with 12 healthy female participants using an 8-hr nighttime sleep recording. Emotional tone was evaluated by five blinded judges and final valence and intensity scores were obtained. Emotions were mentioned in 80.4% of dream reports. The voltage, number, density and duration of facial muscle contractions were greater for the corrugator muscle than for the zygomaticus muscle, whereas high positive emotions predicted the number (R 0.601, p = 0.0001) and voltage (R 0.332, p = 0.005) of the zygomaticus. Our findings suggest that zygomaticus events were predictive of the experience of positive affect during REM sleep in healthy women.

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