"pertaining to or involving sleep," 1690s; see somni- + -al (1).
Entries linking to somnial
somni-
before vowels somn-, word-forming element meaning "sleep," from combining form of Latin somnus "sleep, slumber," from PIE root *swep- "to sleep."
-al 1
suffix forming adjectives from nouns or other adjectives, "of, like, related to, pertaining to," Middle English -al, -el, from French or directly from Latin -alis (see -al (2)).