ETD

• Dream enactment
• Hemispheric asymmetry
• Motor lateralisation
• REM Sleep Behaviour Disorder (RBD)
• Video-polysomnography (vPSG)

REM Sleep Behaviour Disorder (RBD) provides a unique model to investigate the neural control of movement during dreaming, as the loss of physiological muscle atonia in REM sleep allows dream-enactment behaviours to be directly observed. Previous actigraphic and parasomnia studies suggested that motor activity during sleep may shift towards the non-dominant hand, indicating a reorganisation of hemispheric control across vigilance states, but this had not been demonstrated in RBD. In this retrospective video-polysomnographic study, 71 patients with RBD (22,066 manually annotated upper-limb movements) were analysed. During wakefulness, right-handed patients exhibited longer right-arm movements, consistent with expected motor dominance. In REM sleep, this pattern inverted: right-sided movements became shorter and less frequent, while left-sided movements predominated, particularly for complex gestures. Jerk-like movements remained symmetrical, with a mild rightward predominance when all events were pooled. Among right-handers, 72% displayed a leftward predominance during REM, which increased further when considering only complex movements. The side affected by parkinsonism did not abolish this reversal. Dream-enacted movements therefore exhibit a mirror-like inversion of waking motor dominance, suggesting transient redistribution of cortical control and hemispheric balance during REM sleep, providing new insight into the neural mechanisms underlying motor expression in dreams.