It takes a little of her sanity/memory/grip on reality every time. I think one of my quibbles with the book's worldbuilding is that it really isn't evident that this is what is happening until she actually says so, which is something like 90% of the way through the book. She gets severe headaches and gets sick every time she uses it, and every time she comes back to the bridge it's a little more decrepit, so I assumed it was chiseling away at her physically and her mental health issues were mostly because of other reasons, but it also wrecks her mentally as well.