Could this be a problem?

I'm getting a corn snake in a few days and the humidity in my corn snake tank stays at about 40 all throughout the day, and I use EcoEarth as a substrate lining the tank, as well as a light bulb for the warm side of the tank. I was wondering why it was the humidity drastically rises whenever I turn of the lamp at night? It ends up reaching about 60 if not 70 at night. So I was assuming it was because of the substrate I chose to use? I know they don't need that much humidity unless they're in shed so I was curious to know if this could potentially become a problem in the future? Any advice is much appreciated

The humidity rises at night because it is cooler with the lamp turned off. The lamp dries the air.

Look at the inverse relationship between room temp (blue) and humidity (green).

As nanci said, the humidity rises at night because the light isn't drying out the air. As to the high humidity, you are more than likely correct in your assumption it's the eco earth. Stuff is excellent for providing humidity in situations that need it. As for the health of the snake, corns are fairly hardy and live in most temperate climates so 40-60 is pretty much the "normal" for them. Constant exposure to higher or lower levels can lead to different complications, especially if temps aren't kept up to par... but I'm not sure if I would be too worried about a slight night time increase. As long as it doesn't get above 65-70 too often/long. (as you can see in Nanci's charts even her humidity spikes above 60 from time to time).

I know you're considering getting a UTH and thermostat from reading (and replying to) your other thread. If you do that, you might want to find a way to drop that humidity because the UTH wont dry out the air like the bulb is doing during the daytime. Shouldn't be too hard depending on the enclosure, usually you could either let the eco-earth dry out a bit more, change substrates, or even something as simple as increasing ventilation and airflow.