It's basically snap chatting while asleep. It's just like sleepwalking but instead of walking, you're Snapchating.
"Yo, did Megan send you a picture of her feet last night?"
"Yeah, she was sleep snapping"
Someone who unconsciously uses snapchat in their sleep and has not got any control over what it said or being done. Very rare and happens only to people who are experiencing major backstabbing.
" This girl is opening my messages and ignoring them so I asked what's up? She replied that she had for a sleep snapping syndrome"