Why is window reveal damp, and what can I do?

[JP_]

One of the places I could not insulate (due to cost, didn't want to install new windows etc) was the window reveals in my bays.
To recap - 1930s solid wall construction
Overall is dry inside.
No obvious sign or leak in roof area or loft.
Pebbledash is falling off in places, will be replacing all with new render in future - could this be a problem? Although, I can't see how water would get in behind it and run down due to roof overhang.
Condensations forms over night on the window, I always open windows in mornings to help dry out

Darker damp area outlined in case not clear in this photo. Already had some black mould there this month so not necessarily staining. But could be.

 

[pilsbury]

I’m no expert, but I’m assuming it’s condensation. Warm moist air on a cold surface. But why? Why do you have condensation all over your windows? My double glazing doesn’t condensate like that. I suppose I’m suggesting you have a lot of humid air in your house.

 

[JP_]

It's just the bedroom at night - you know, lots of heavy breathing. We live on a busy road so don't have trickle vents or leave the window open.

Condensation certainly makes more sense really, as I really can't see how water is getting in, and similar both sides.

Might look into some sort of ventilation then ... Or just live with it.