Really atmospheric game, great job!
The manual breathing mechanic is very unique, never seen that before. Took me some time to figure out, then I got it consistent to full light, but later struggled to get anything more than dim light. My timing probably just got worse, but I think some more feedback what the timing should be would be helpful.
I really appreciate how you avoided a HUD by showing health with the brightness of the character and put the tutorial as markings on the floor. It makes everything more immersive!