The leaves of a snake plant are hardy in healthy condition. Any sign of snake plant leaves curling inward indicates underwatering.
If your plant is not watered adequately then its leaves start curling. It means that the moisture content in the leaves is decreasing.
The best way to save your snake plant from curling leaves is to water adequately so that its leaves gain water content in its leaves. I have written a guide on how to water snake plants.
Why snake plant leaves curl?
As we know that snake plant is a succulent plant. It stores water in their fleshy, thick succulent leaves. When the plant is too dry or moisture content is very less, the leaves of snake plant starts curling.
The major cause of snake plant leaves curling is underwatering. The other causes can be low temperature, insect infestation, and fungal infections.