Etymology

From a parallel i-stem form to Proto-Baltic *sapnas, from Proto-Indo-European *súpnos, *swépnos, *swópnos ‎(“sleep”), *swópniyom ‎(“dream”), from a stem *seup-, *swep, *sup- ‎(“sleep”), from *seu- (meaning indeterminate, perhaps “to lay,” “to put”) with an extra element -p. In Baltic, the words for “dream” and “sleep” merged, and the initial consonant was simplified (sw > s). Cognates include Lithuanian sãpnas, dialectal sãpnis, sapnỹs, Proto-Slavic *sъnъ < *supnu (Old Church Slavonic сънъ ‎(sŭnŭ), Bulgarian сън ‎(sǎn), Czech, Polish sen), Old Irish sūan, Cornish hon ‎(“sleep”) (< *swopnos), Old English swefn ‎(“sleep, dream”) (< *swepnós), Hittite šup- ‎(“to sleep”), Sanskrit स्वप्नः ‎(svapnaḥ, “sleep, dream”), Ancient Greek ὕπνος ‎(húpnos, “sleep”) (< *supnos), ὕπνιον ‎(húpnion, “dream”), Latin somnus ‎(“sleep”) (< *swepnos, *swopnos), somnium ‎(“dream”), sopor ‎(“deep sleep”).^[1]