In The Elder Scrolls V: Skyrim for the PC, PS3, and Xbox 360, vampires need to feed in order to reduce the stage of vampirism that they’re on. Reaching stage 4 of vampirism makes most humans want to kill you on sight, so feeding is necessary at this point if you want to interact with humans in any way.
To feed, you need to sneak up on a sleeping human and use the Pickpocket option. At that point, you’ll see an option to suck their blood. Feeding will always reduce the vampirism stage to stage 1 no matter what stage you were on before.
Tips for successful feeding