Waves on the ceiling due to increased room humidity?

Hello! In our new building we have noticed the following problem: In the current season, with increased use of the lights, we see strange waves or "imprints" of the rafters on a ceiling. These are actually only visible when the light is on, hardly visible in daylight. The ceiling has counter-battens and is covered with a paneled ceiling. On top of that, there is textured wallpaper applied. What could be the cause? Possibly because we have increased room humidity and have already been running an electric drying device. The room humidity was measured at about 55-60% (overnight), during the day about 48-55%. Thank you very much for your help!

I live in an old building and also have waves on the ceiling. Actually, it is fine, only the appearance is not really great. The floor and the walls could also be redone. The most important thing for me at the moment is the floor. Now my question: do you really have to start with the ceiling and work your way down, or can you start with the floor? The floor is simply my top priority and I cannot afford to have everything done at once.

There are very nice possibilities to design ceilings. The following online shop offers such ceiling moldings: link durch Moderation entfernt; Bauexperte