I recently read the hulk#1 and noticed he changed in the hulk at night...like a warewolf lol. It seemed then he turned back into banner when the sun light hit him. When did this change? Was it just a first issue kind of thing?
A quick search reveals that he was cured of the Hulk early on, but decided to dose himself with a gamma ray machine to change into the Hulk with all of his intelligence, at the cost of leaving Banner sick and weak when transforming back. After repeated and frequent use of the device, the Hulk changed into a rampaging monster, and Banner began changing whenever he got angry, resulting in the Hulk as we know him.
'Course, whether that information is accurate or not, whether due to outright false information or some retcon is anyone's guess. If you want a second opinion, summon TheAcidSkull. He'll settle the matter definitively.