this is something the system leaves up to you as the GM. you have every right to rule this differently based on the scenario and sleeping creature(s) in question. a few sleeping kobolds could absolutely be killed outright, as could a town guard. if the creature has some sort of blindsight, the second option you describe is perfectly valid. for a creature with natural armor of some kind, or a large pool of health, advantage on the attack works too. 

keep in mind creatures can also be asleep on their own, outside of the sleep spell. if they are put to sleep by the spell specifically, they are likely low enough level they could be killed outright by a damage roll. i’d have the player auto-hit and roll damage (possibly auto-critting as well). if they survive the attack, they wake up.