Hunting Guardians at night is a recent option that allows players to capture guardians in a nighttime setting. Guardians in this mode are still active, and still break as normal depending on the location you are hunting.
What differentiates hunting in the day from hunting at night however, is that there is always a different mechanic from hunting in the day.
Guardians here have more HP, taking more than 2 shots to capture (some guardians can take 10 shots before capturing).
Guardians do not break.
Green Guardians: Upon capturing, these Guardians supercharge the next shot, allowing you to capture the next Guardian in one shot.
Snowman Break: Randomly on the map, there will be a Nine Lives Lenny snowman; shooting it five times will cause a 'snowman break', turning all four guardians on the map to freeze, allowing you to catch them all in one shot.