The gradient that is mostly no gradient
Every other animal in the reptile room wants a hot end and a cool end; the crested gecko wants a mild, even coolness. He comes from the humid forests of New Caledonia, where nights are cool and days are gentle, and his whole enclosure should sit between 72 and 78°F. The only gradient he needs is the one physics provides for free in a tall enclosure: the foliage near the top runs a degree or two warmer than the substrate at the bottom, and he climbs or descends to fine-tune. There is no basking spot to build, no heat mat to control, and no hot end to measure. If your home sits in the low-to-mid 70s, your gecko's heating setup is already finished — what he actually needs from you is a thermometer and vigilance about heat, because his tolerance runs out fast in the other direction.
Heat is the hazard, not the cold
Sustained temperatures above about 82°F stress a crested gecko, and the mid-80s and beyond can be fatal surprisingly quickly. A lethargic gecko sprawled flat on the enclosure floor or pressed against the coolest glass during a hot spell is a gecko in trouble. The common causes are mundane: a vivarium in direct sun from a window, a room that bakes in the afternoon, a heat mat added 'just in case' by a keeper used to leopard geckos, or a hot enclosure stacked under a bearded dragon's lamps. Keep the enclosure out of sunbeams, know which room in your home runs coolest in summer, and be ready to move him there — or run the room's air conditioning — when a heatwave lands. In winter the opposite worry is mild: a dip into the mid-60s at night is well within what this species handles in the wild, and a gecko who eats less and moves less in a cool room in January is usually just being seasonal.
A tall enclosure does the fine-tuning
Because his gradient is vertical, the enclosure should be taller than it is wide, filled with foliage, cork rounds, and branches at several heights so he can choose his level. Misting in the evening raises humidity and briefly cools the leaves, and many crested geckos drink the droplets as they settle for the night. A small digital thermometer at mid-height tells you the number that matters; one at the top confirms the gentle warm layer stays gentle. Food plays a part in reading him, too — a gecko at a comfortable temperature eats his prepared diet readily and hunts insects with enthusiasm, while one who is chronically too warm goes off food and loses the plump look around his hips and tail base. Weigh him occasionally if you are unsure; the trend line is a better witness than any single evening's appetite.
When cool-keeping becomes a vet question
Most crested gecko temperature trouble is heat stress, and a gecko found limp, unresponsive, or twitching after a hot afternoon is an emergency for a reptile vet that day — cool him gently towards room temperature on the way, never in cold water. The slower presentations deserve a visit as well: steady weight loss, a tail that stays thin, repeated poor sheds with retained skin on the toes, or a gecko who has stopped eating entirely for weeks in a room you have verified is in range. Take the enclosure's temperature pattern with you — daytime high, night low, and the hottest the room reached — because a vet diagnosing a gecko from a heatwave week needs exactly those numbers. And when the forecast turns hot, put a follow-up question to KinsLe's Kimmy; she answers using the KinsLe Answers library.