This is a specific type of Transformation Cause that uses the shift from day to night as its trigger. This is particularly common with modern depictions of werewolves, since being cursed to transform into a wolf-monster one day of the month doesn't seem like much of a curse. If a character has this sort of transformation, you can usually expect it to be mystical in nature, tied to the ancient perception of night as a fundamentally different time than day (rather than, you know, cosmic objects being in different positions). If not, then it might be a form of therianthropy based on a nocturnal animal, like the aforementioned wolves.
Technically speaking, any being that transforms as a result of the full moon could also be said to fit this description in a literal sense, but they are Not Examples as the night isn't the catalyst in those cases.
Compare and contrast Nice Day, Deadly Night, which is when places rather than people become more dangerous at night.
- The commercial for Need for Speed: Underground shows someone locking their stock looking Nissan Sentra in a garage as night falls. Then the car starts to develop a body kit with custom hood and spoiler while the paint job turns into something more elaborate. Basically, it looks like the car just underwent the game's customization system.
- This is the trigger for the tile character's transformation in Maze. The change from day to night triggers Maze's Gender Bender transformation from female to male with an accompanying change in personality and the inverse holds true, triggering the transformation from male to female. It triggers even deep underground where there's no way to know where in the day/night cycle they are.
- Inuyasha has the half-demon Gyu-oh. While the other half-demons look strange and keep their looks all the time, Gyu-oh is a pure human during the day and a pure bull-youkai at night. Over time, these changes have driven him insane.
- In the comics based on Buffy the Vampire Slayer a new breed of vampires emerges, which are not only much stronger than the old vampires, but also immune to daylight. And they can change their shape at night. It is a voluntary change of form, but they can only do it at night. Two of them, Vicki and Trevor, like the form of a giant bat.
- Ghost Rider originally transformed like this at night before Retcons enabled him to bring the Flaming Skull out more often.
- The Incredible Hulk originally used this as the cause for his transformation, before the more well-known anger cause was fully established. Several Hulk personas like Grey Hulk (Joe Fixit) and the Immortal Hulk also use it, to hearken back to the character's roots. The reasons have never been explored much, but regardless, the night is ''his'' time.
- In Shrek, Princess Fiona is under a curse where she changes from a human to an ogre when the sun goes down. She's very self-conscious about her appearance, so when Donkey finds out, she makes him promise not to tell Shrek. Near the end of the movie, Shrek finds out about the curse, but since he is an actual ogre, he thinks Fiona is beautiful in her ogre form. He marries her, and as a result, Fiona remains an ogre for the rest of her life.
- In Ladyhawke, Isabeau and Navarre have been cursed by the villain so that she's a hawk during the day and he's a wolf during the night, so that they're never human at the same time. The breaking of the curse involves an eclipse, which blurs the shift from day to night and allows them to both be human at the same time.
- Played with in Pirates of the Caribbean: The Curse of the Black Pearl: the titular curse is in effect all the time but at night, specifically under the light of the moon, the cursed pirates are revealed to be walking skeletons, while they look human the rest of the time.
- N. K. Jemisin's Inheritance Trilogy: In The Hundred Thousand Kingdoms, the Physical God of Darkness Nahadoth is enslaved and can only exist in his divine form at night. By day, he's an ordinary human with no memory of his true self.
- Invoked in the story of Sir Gawain and Lady Ragnelle. Lady Ragnelle was cursed to be hideous but upon marrying Gawain, the curse was half-broken. Ragnelle said that she would either be beautiful during the day and hideous at night, or hideous during the day and beautiful at night, and Gawain had to choose. Ultimately averted because Gawain allowed her to choose, breaking the curse completely.
- The X-Files: In the first season episode "Shapes", a rancher who killed a Native American man insists he fired at a monstrous animal, and is himself later mauled to death in a mysterious animal attack. Tribal elder Ish tells Mulder that he believes (correctly, of course) that the attack was the work of a Manitou, supposedly an evil spirit that transforms its human host into a wolf-like beast. Presumably anticipating the obvious comparison, Ish specifically states that the Manitou changes by night, not by the full moon.
- In a novelle, Agent Skinner meets an old friend he knew from the war in Vietnam. There he was bitten by a wererat and now turns into such a creature every night. He lives in a poor area and kills the homeless every night until Skinner kills him.
- As mentioned above, some versions of werewolves. For instance, in certain regions of Brazil werewolves are said to transform at midnight on Fridays.
- In Minecraft, spiders go from neutral creatures who only attack if provoked, to full-blown evil animals who'll attack as soon as they see you at night.
- Sonic Unleashed: Sonic transforms into a 'werehog' at night due to Dark Gaia's corruption, which also comes with a separate style of gameplay than his normal form.
- Trials of Mana has half-beastman Kevin, who transforms into a werewolf form at night which increases his attack. This happens to other beastmen too.
- This also happens to the werewolves in Ogre Battle 64.
- Inverted for the Pokémon Shaymin, who can transform into its Sky Forme during the day but changes back when night falls.
- In Knight Lore, Sabreman is transformed every night, having been cursed with lycantropy in the previous game.
- A reccuring creature in the Grow games and others Eyezmaze games is a creature whose form is different during the day and the night. During the day, it's a white blob who sleeps by default but can be awaken by an alarm clock, it's soft body can grow limbs or a long neck. During the night, the creature is awake and gets hands, wings and horns and is half-green, half-yellow though the colors may change depending of the game. The day form is called "Rotta" and the night form is called "Pierre".
- Unsounded: Duane Adelier's mind travels into the Background Magic Field while the sun is down, leaving his body controlled only by its Horror Hunger. It's a side effect of whatever power animates him, but the reason for its relationship to the sun is unknown.
Rahm: Adelier's a monster rotting in body and mind, with the Khert every night trying its damnedest to pull him apart and send him on.
- Inverted in Gargoyles. The Gargoyles turn to stone in the day time, and come alive at night. It functions like an extreme forced sleep cycle. It activates a Healing Factor and generally recharges them, but also leaves them vulnerable if they're not at home base, as they get instantly petrified mid sentence wherever they are for at least 12 hours.
- The Venture Bros. has minor characters Lady-Hawk and Lyndon-Bee Johnson, a rather blatant Shout-Out to Ladyhawke. She's a woman in the day and a hawk at night, and her husband is a bee in the day, and a man at night and they are cursed to never be human at the same time.
- Moonshadow elves in The Dragon Prince can change at night under a full moon. Then their shapes become strangely ghostly, but their powers increase. Particularly powerful moonshadow elves can change into this shape without the full moon, but they can only do it at night.