Need a little help here

Why does my 13 months-to-be baby start crying/wailing as soon as he wakes up? I start to put him to sleep by 750pm the night before, and he falls asleep by 815pm. By 2+am, I noticed that he will start stirring a bit, or tossing and turning, daddy manage to sooth him to sleep, but then stirs again by 4+ and 5+ he will be up and crying. Advise, any mummy?