Thermal gradient

A thermal gradient is the range of temperatures that runs from one end of an enclosure to the other, created by placing a heat source over only part of the tank rather than warming the whole space evenly. The idea comes from how reptiles behave in the wild, moving between sun and shade to keep their body temperature in a workable range, since they cannot generate their own heat the way mammals do. A properly built gradient gives the animal that same choice inside a glass or PVC box.

Building one starts with placing the basking bulb, heat mat, or heat panel at one end of the enclosure only, then measuring both the hot end and the cool end with separate thermometers or temperature probes rather than relying on a single reading or the dial on the heat source itself. Species have different target ranges, since a bearded dragon basking spot runs much hotter than a crested gecko's warm side, so the right gradient depends entirely on what is being kept. The cool end should still sit within a safe range, not just be colder than the hot side.

The most common mistake is heating the entire enclosure evenly, either with an oversized heat source or with heat cable run the full length of the tank, which removes the animal's ability to cool down and can lead to chronic overheating or stress. The second common mistake is measuring air temperature in the middle of the enclosure and assuming both ends match it. Two thermometers, one at the basking spot and one at the cool side, are the minimum for checking that a gradient is actually working.