Lewis & Short

somnĭālis, is, adj. [somnium], dreambringing, an epithet of Hercules, Inscr. Fabr. p. 692, 130; Inscr. Rein. cl. 10, 4.
Hence, adv.: somnĭālĭter, in a dream (late Lat.), Fulg. Myth. 2, 17 dub.