Measure the floor, not the air
Ask a leopard gecko where his gradient is and he would point at the ground. This is a desert-edge ground gecko who spends the day in a burrow or rock crevice and warms himself on stone that holds the day's heat into the night, so belly temperature is the temperature that runs his digestion, his appetite, and his immune system. The floor inside his warm hide should read 88–92°F. The air at the warm end sits lower, in the mid-80s, and the cool end of the enclosure should fall to 75–80°F at floor and air level alike. Keepers who hang a thermometer halfway up the glass are measuring a zone the gecko never visits, which is how a setup can look right on the dial and be wrong everywhere the animal actually lives. An infrared gun swept across the floor tells the truth in seconds.
Three hides across three zones
The classic leopard gecko layout is three hides, and each has a thermal job. The warm hide sits over the heat source — usually a heat mat under one end of the enclosure, run on a thermostat with the probe on the floor inside the hide, holding that 88–92°F surface. The cool hide sits at the far end, where he retreats to sleep the day away at 75–80°F. Between them goes the humid hide, lined with damp moss or paper towel: not a temperature zone but a moisture one, parked in the middle ground so its humidity survives without soaking the whole enclosure, and it is where he will go to loosen a shed. Water sits at the cool end. The mistake to avoid is crowding everything warm: a heat mat covering half the floor erases the cool end, and a gecko with no cool end becomes a gecko who stops eating, sheds badly, and soaks in a water dish never designed for it.
Digestion and sheds are the daily report
A leopard gecko whose warm floor is right eats with enthusiasm, digests within a day or two, and produces firm droppings with a clean white urate. A gecko whose warm end is quietly running cold goes the other way: meals refused or eaten slowly, undigested insects visible in the droppings, long hours pressed flat on a warm spot that never quite delivers. Sheds report on the rest of the setup — a clean shed eaten overnight is normal, while retained skin clinging to the toes points at a humid hide that dried out or a cool end that ran too dry and cold. Toes matter here; stuck shed tightens around them over successive sheds and can cost the tips, so a gecko who keeps failing to clear his feet is telling you the middle zone needs attention even if the two ends read perfectly.
When it stops being a thermostat problem
See a reptile vet when a leopard gecko loses weight steadily — the tail is his fuel gauge, and a tail that has gone thin and creased is a gecko spending reserves — or when he stops eating entirely for weeks outside any breeding-season fast, develops a swollen or soft jaw, or keeps retained shed on the toes despite a working humid hide. Bring your floor readings: warm hide surface, cool end, and the night-time low. Those three numbers let the vet separate a husbandry gap from a medical problem faster than any description. Small adjustments, each left a day to settle before the next, beat big swings, so put a follow-up question to KinsLe's Kinny; he answers using the KinsLe Answers library.