Night 6 is the "Nightmare" level where the difficulty spikes significantly because you are dealing with all the standard animatronics until 4 AM, at which point takes over the entire house. Here is a solid write-up and strategy to help you survive: Phase 1: 12 AM to 4 AM (The Gauntlet)

You must check the doors constantly. Run to a door, listen for breathing for at least 2–3 seconds.

During these hours, you face Nightmare Bonnie, Chica, Freddy, and Foxy. Speed is everything.

At 4 AM, all other animatronics vanish. You will hear a deep, haunting laugh. Now, it is just you and Fredbear.