Stage of the relationship: Every stage

If you neglect her vagina (see prior point) and then use your "long day" as an excuse to not bring the nookie full circle, you may wake up to being smothered by a pillow.

If the former point doesn't apply, and you're her boyfriend, she'll expect some interaction before you nod off. At the very least, a congratulatory high five for a job well done together. It affirms your bond, and it's an important ritual to maintain each night.