How to stop my Syrian hamster from gnawing bars at night?
That's normal hamster behaviour.
They have to gnaw to keep their teeth short, the things grow continually. Is there anything in the cage for her to gnaw on? Hamsters are nocturnal, they're active at night. Is there a wheel she can run on? Is the cage too small?
Its a sign of boredom. How big is her current cage? Minimum is 450 square inches but bigger is better. She needs at least 5 or so inches of bedding to burrow in. She needs a proper wheel. Large enough so her back doesn't bend. It needs to be solid. No wire or mesh. Rotate toys on a regular basis to keep her interested. I always did an entirely new setup each time I cleaned my hamsters cage. You can always get her a tank. She still needs everything I listed above but if you get a tank there will be no bars to chew. You could get a bin cage as well and just don't put mesh on the sides.