what are the top reasons that baby wakes up in the middle of the night?

On top of the reasons given above, babies might wake up in the middle of the night because of over-excitement in the day. For example, my baby niece, if she was very "social" (e.g. being around too many people at a gathering) in the day, she'll end up waking up a few times at night. After being coaxed and comforted a few times, she sleeps peacefully... like a baby haha.