"sleepy, drowsy, inclined to sleep," mid-15c., sompnolent, from Old French sompnolent (Modern French somnolent) or directly from Latin somnolentus "sleepy, drowsy," from somnus "sleep" (from PIE root *swep- "to sleep"). Respelled 17c. on Latin model.
It forms all or part of: hypno-; hypnosis; hypnotic; hypnotism; insomnia; somni-; somnambulate; somniloquy; somnolence; somnolent; Somnus; sopor; soporific.
It is the hypothetical source of/evidence for its existence is provided by: Sanskrit svapnah, Avestan kvafna-, Greek hypnos, Latin somnus, Lithuanian sapnas, Old Church Slavonic sunu, Old Irish suan, Welsh hun "sleep;" Latin sopor "a deep sleep;" Old English swefn, Old Norse svefn "a dream."