Night temp and humidity

Is it ok for my tent to be at 15C and 80-90humidity at night with no light no fan or can this be stressful?

Personally, I think the RH is up there.
I would keep your fan going, even if you have to put a heater in there, or get a dehumidifier for the room.

59° (15c) is a little cool I would try and stay above 60°. 80-90% rh is bad, you will most likely have mold problems if you dont correct it. I try and stay under 60% and have a 35 pint/24hr dehumidifier on hand just in case. Also run your exhaust and circulation fans 24/7.