They will dig into the substrate to either hide or go to where the moisture is due to low humidity. If you find you frog is hiding in the substrate, check your hydrometer to ensure it s functioning correctly since they are not the most accurate things. Also take humidity readings for tree frog higher in the tank, and not hear a heat source such as a light or side placed UTH.

If the humidity is fine, then they are hiding in the substrate due to not having a place they feel hidden or safe, higher up in the tank.

Now, is this always true? Sometimes one of my WTFs would dig in under a log while the other three were all in their normal sleeping spots, high up in the plants. So, go figure :-)