Where does “сонливость” come from?
сонливость (Russian) comes from Russian сонли́вый, from Russian сон, from Proto-Slavic sъ̀nъ, from Proto-Balto-Slavic supnas, from Proto-Indo-European supnós, from Proto-Indo-European swóp-r̥ ~ swép-n̥s, from Proto-Indo-European swep- — to sleep.
сонливость (Russian): sleepiness
Ancestry of “сонливость”, step by step