Sleep over
Definition 1 of 2
Pro Tip 1/3Social Event
Often used when discussing plans for casual social gatherings. 
Are you going to sleep over after the birthday bash?
Pro Tip 2/3Children's Parties
Commonly refers to children or teenagers staying at a friend's home. 
My son is excited to sleep over at his friend's place this weekend.
Pro Tip 3/3Informal Invitation
Used when inviting someone to stay at your home, in an informal setting. 
If it's too late, you can sleep over at my apartment.