Why do some people like to leave parties early?

My homebody friend was already to go home at 8:30pm. What?? She has nothing to do and we convinced her to stay. This party is going on ALL NIGHT. A friend's bday party. We like to have fun. I do! My friends and I were laughing and having fun. I'm so not ready to go home at all. It's like almost 11pm where I live and we're still partying. I'm having fun. My friend wants to go home. She ALWAYS does. She is always in bed by 9:30 or 10pm...even on weekends. She's 24! We're young! We should be getting crazy!!