The sleep spell that can put you to sleep for longer than the length of an average combat so you are at their mercy. If you have few enough HPs for sleep to affect you, the automatic critical they can get (Unconcious condition, PHB 292) will likely kill you.
Alternately since it takes damage or someone spending an action to slap or shake you awake they can probably carefully tie you up/manacle you, toss you into off a cliff, take away all of your weapons, put you in a noose, or other thing that doesn't jostle or hurt you prior to it going off.
Yes, the spell doesn't kill you itself, but it does completely take you out. Just like petrification can turn you to stone but "you're not dead".