Negative facial expressions and frowns have been studied (albeit more rarely than smiles) in fetus' and neonate' sleep, but they have not been investigated during adult sleep. Video polysomnography (including corrugator muscle electromyography and face-focussed video) was used to study negative facial expressions in sleeping adults, including healthy subjects and patients with/out parasomnia. Frowns were observed during sleep in 89/91 (97.8%) adults during normal (29 healthy subjects) and abnormal sleep (29 patients without parasomnia, 15 patients with disorders of arousal and 18 patients with rapid eye movement [REM] sleep behaviour disorder [RBD]). In healthy subjects, the following events occurred in decreasing frequency: isometric corrugator activations, brief frowns, and then prolonged frowns and raised eyebrows (both similarly rare). Frowns predominated in REM sleep, and had a lower frequency in non-REM sleep. In healthy subjects, frowns were elementary and not associated with other face movements to the point of composing negative expressions. In contrast, frowns were occasionally associated with overt negative facial expressions in REM sleep in patients with RBD and a young patient with night terrors. They included mostly painful expressions and rarely sadness and anger in connection with apparently negative behaviours (shouts, painful moaning, and speeches). Frowns persist during normal sleep (mostly in REM sleep) in adults, but overt negative facial expressions are restricted to patients with parasomnia. Whether elementary frowns translate a negative dream emotion should be determined, but overt negative facial expressions during RBD could be used as a direct access to dream emotions.