Let us address the more important issue first. Hopefully, she is on birth control because, I think the real problem with her "sleeping over" is that she isn't sleeping. If the boyfriend is leaving after the summer, then this situation will run its course. If he is back permanently, then it may be time for her to get her own place. 18-24 is the age when the waters of independence are tested. If you try to hold her back, she will rebel. She is not going to spend her days hanging out with you when she can be with a boyfriend who makes her feel like she is on a cloud. You can't compete with that emotion. Just make sure she doesn't prematurely start a family. Letting go of your child is tough. You are in the beginning stages of her leaving for good. My advice is to take her to lunch, and start working on your adult relationship with her. Your time with her home is short.