What could be the reason why you wake up too early in the morning? Not enough internal melatonin?

Depression, diabetes. Well, if you are just lying there it's likely depression and/or anxiety. If you wake up a lot to empty your bladder think diabetes or sleep apnea. If you are coughing you probably have asthma. When my wife was 34 she woke up a lot because the baby was crying. I slept right through that.