when you are asleep and someone you have never been intimate with before wakes you up and starts kissing you and generally leads to engaging in sexual activites
"dude i was falling asleep and then amy came into my room and sleep assaulted me, it was totally hot"
when youre partner(another boy or girl, can be gay or strait) falls asleep, but you can have sexual intercourse with them. Only works if the one doing action is a boy.
Last night I was super horny, so before Linda fell asleep she said I could sleep assault.