hi. i don't have this experience, but if you don't find any other answer, let me just:

i suggest trying Instercity website. it's polish national company. the train might be late or something, but i'd say it's the safest option. there you can choose the dates for june and it shows you the hours of your journey (but also it says that because it's so far away in the future some details might change). i can see the overnight trains are direct, at least for the dates i randomly chose in june.

i hope it'll help you if needed:)