Bedtime

This seems strange, but my daughter somehow really "gets" bedtime. I put her in her pack n play and she lays there until she falls asleep. It only happens at night. I can't put her down during the day at ALL. But the strange part is if she's crying and I put her to bed, she stops crying. Of course she's been changed and fed, but I don't know why she's crying. I just think it's weird that the crying stops when I put her to bed. Any baby psychology insight?