tired little one

She gets so tired sometimes but forces herself to stay up... lol I don't understand why babies do that. I feel like telling her to just fall asleep!!! The other day, I tried to put her down and leave the room, but she kept crying as soon as she spits out her binky!!! So I just took her to where I was scrapbooking. She was playing around for a little while, and all the sudden it was really quiet, so I looked to the side and she was sleeping sitting down...