Sarah's Mind is a dungeon on the Heroic Path.
To access it, you must complete Valerie's Mind and choose to use Boost on her.
Next, when you encounter Sarah in the hallway, talk to Jane and then to Dr. Tiff in order to enter her mind. Which is literally on fire. Everything is on fire here. Everything. Including the enemies.
While you don't lose HP by being in here for too long, the enemies packs more of a punch compared to in Val's Mind and as such you should advance through it as soon as you can. That said, it's not a terribly hard dungeon.
Dungeon Walkthrough
In the first room, head north. You'll see what looks like a large room. Enter it.
In the second room, head to the bottom right corner and touch the Sigil there. This will thrust you into a boss battle, which is extremely tough compared to the boss in Val's Mind. Be prepared.