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)

Don't let the "Freddles" accumulate. Check behind you every couple of door rotations. If there are three, you’re in immediate danger.

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

When you reach a door, if you think he's there, flash very quickly. If he’s at the end of the hall, close the door immediately. Critical Tips for Success