Thermoregulation

Reptiles and amphibians are ectotherms, they cannot make their own body heat, so they raise and lower their temperature by moving around their environment, basking to warm up and retreating to shade to cool down.

This is why a single enclosure temperature is never enough: the animal needs a range to choose from. Give it a warm end (with a basking spot) and a distinctly cooler end, and it will self-select what it needs through the day. Get the gradient wrong and digestion, appetite, immunity and activity all suffer.

In practice: measure the temperature at both ends, at the animal's height, with a probe, not a stick-on dial. Logging those two readings over time is the single most useful habit in reptile keeping.