Sleeping together and having sex. Not to confuse the term sleeping together but not having sex with sleeping together AND being in a sexual relationship.
Similar to sleep walking, sleep sexing is when two people copulate while being asleep. Usually they wake up in the middle and figure out what's going on then after it's over fall back asleep. Later on they might even think it was a dream.
boy: so I had this amazing dream that we had sex this morning!