Hi Creister. I'll take a stab at it. If the relative humidity goes above 65-70+%, leave the fan on so you don't develop fungal infections on your seedlings. (Fungal infections can have other causes too). However, it depends on what you are growing. I leave the fan on at night for my CAM plants that take in O2 during the night because their stomata are open, quite the reverse of most plants, regarless of what the humidity is.