High humidity in bedroom and condensation
In general, high moisture levels can be reduced by exchanging the room's air only during night, best time in the morning before sun rise.
Best method would be to open 2 windows at 2 different walls of the apartment/building, west and east sides would be the best option. 

If there is a noticeable air flow, just 1-3 minutes are enough and should not be exceeded. 

That way statistically colder and dryer air is replacing the warmer air without cooling down walls, furnitures etc. 

This method is extremely counter-intuitiv since most people do not want to get colder air into their home. 

But warming up dry air needs only a fraction of the energy needed to heat up moist air.

Without this air exchange the building's material, furnitures and interior have to be heated up above the dew point what needs much more energy.   

Since the humidity level in the mentioned bedroom is reduced during daytime, it could be due to the regime "airing during daytime, reducing temperature during nighttime", which is likely causing high humidity levels in the nighttime.
If the hull is not tight enough, statistically moist air comes into the building, since normally the wind blows stronger during daytime. Could be checked via candles, smoke, thermography or 30cm long hanging strings scotched to door and window frames.