Many fans of the Incredible Hulk believe Bruce Banner's transformation is governed entirely by rage, but it's actually the result of multiple factors. During the course of his adventures, Banner has learned that radiation, magic, and even willpower can cause him to change (or not) into his alter ego.
At one point, Banner began changing into a gray-skinned Hulk at sunset (the original trigger for his transformations from The Incredible Hulk #1) and reverted to himself at sunrise. Later, however, fans learned that along with the night, the Hulk was affected by moonlight in the exact opposite way it affects werewolves!
Hulk learned this curious fact in The Incredible Hulk #333. By this point, the Gray Hulk had been around for a while and was obsessed with gaining dominance over Banner. Much smarter and more amoral than his previous green-skinned form, the Gray Hulk actually fell in with his old enemy the Leader who discovered something strange about this new Hulk’s biology. In an odd dream sequence, the Leader’s mind reached out to the Gray Hulk and revealed that moonlight had a curious effect on the Hulk - since moonlight is basically reflected sunlight (which triggered the change back to Banner), the more moonlight the Hulk absorbed, the more influence Bruce Banner’s persona had over him. Thus, during nights of the full moon, the Gray Hulk acted kinder and was noticeably more reasonable than other nights. On the other hand, on nights of the new moon when there was no residual sunlight to affect him, the Gray Hulk’s cruel, greedy persona was in full control and could even subliminally influence Banner’s actions.
It was an interesting inversion of the usual werewolf story as a full moon - which turned werewolves savage - actually made the Hulk less dangerous than he normally was. The Hulk attempted to use this knowledge to his advantage, reasoning that he would be at his peak during nights of the new moon and could truly cut loose. Shortly after this revelation, however, the Hulk experienced another major change in his transformations when he was summoned to the sub-microscopic world of K’ai where his green-skinned form (with Bruce Banner’s mind) was once a great champion.
After helping some sorcerers take over the world, Hulk received an unusual gift; the K’ai sorcerers were able to suppress Bruce Banner’s persona and give the Hulk total dominance, allowing him to remain the Hulk during the night and day. Sunlight continued to hurt the Hulk physically, requiring him to remain covered up or indoors during the morning and afternoon, but he was finally free to create his own life - which he did by becoming the Las Vegas leg breaker “Joe Fixit” for several months. Banner’s personality eventually reasserted itself, however, and the Gray Hulk had to once again deal with living a shared existence. Later, Banner, the Gray Hulk, and the Green Savage Hulk were all merged into one “Professor Hulk” being with his own unique strengths and weaknesses - but for a time, the Hulk occupied a curious position as Marvel’s reverse-werewolf!