Though it has been done before, my first memory of a game with an active day/night cycle was Legend of Zelda: Ocarina of Time.
While running around most outdoor areas, the game's internal clock would cycle day to night and back to day again. Different enemies would appear between day and night, but the biggest change was to see how the world appeared. Enemies became less visible and the draw distance (how far you could see) was affected. Using what we've learned in this chapter so far, let's create a day/night cycle in GameMaker.
We'll start from scratch for this recipe because you only need three things: A room (rm_game), a block object to test your sight (obj_block), and a night cycle controller object (obj_cycle). Give obj_block a simple sprite and place it in random places around the room, but do not assign a sprite to obj_cycle. Also, make sure obj_cycle has a depth with a lower value than everything else in the room.