My reptile stopped eating: can temperature alone explain it?

It is often the first thing to rule out, but it is not a diagnosis. Below its active range a reptile digests slowly and stops feeding to avoid holding undigested food in the stomach. Before looking for more complex causes, verify gradient, basking spot and night minimum with written readings. If the values have been correct for at least a week and the refusal continues, book a consultation and bring the measurement log with you.

Can I use a heat rock instead of a lamp?

In most situations this is not advisable. Heat rocks create a very hot contact point on a small surface and do not warm the surrounding air, so they do not build a real gradient. Many species perceive heat mainly from above and will not move away from a scorching surface until damage has already occurred. An overhead source under thermostat control remains the more predictable choice.

How often should the ultraviolet lamp be replaced?

Follow the lifespan stated by the manufacturer for that model and write down the installation date on the day you fit it. Ultraviolet output falls long before the lamp stops emitting visible light, so relying on the light itself keeps an ineffective source running for months. If you can borrow a radiometer, a check at the animal's real resting distance is the most reliable control.

Does the temperature really need to fall at night?

For most terrarium species a night time drop of a few degrees reproduces natural conditions and causes no problem, as long as it stays above the minimum for that species. The critical factor is the room: in winter an unheated room can pull the enclosure well below that minimum. Measure for two consecutive nights before deciding, and if needed add a source without visible light on its own thermostat.

Non conventional species45 minLevel: IntermediateUpdated on July 30, 2026

Terrarium thermal management: measure, calibrate and check

How to build a correct thermal gradient, which instrument actually measures what, and reference values for the most commonly kept species.

Audience: Pet owners
Species: Reptiles
Scope: Valid everywhere

What you need before you start

Un termometro a sonda digitale con cavo, per leggere l'aria nel punto esatto in cui sta l'animale
Un termometro a infrarossi, per leggere la temperatura delle superfici e del punto di riscaldamento
Un termostato compatibile con la sorgente di calore installata, con sonda posizionabile
Un igrometro digitale e un registro su cui annotare le letture giorno per giorno

A reptile produces no useful metabolic heat: its body temperature depends almost entirely on the environment. That means digestion, immune response, activity and even the ability to respond to treatment all move with the temperature of the enclosure. Many problems blamed on food or stress actually start with a wrong thermal gradient, a thermostat probe placed in the wrong spot, or a thermometer that measures something different from what the keeper thinks it reads. This guide explains how to build and verify a thermal gradient, which instruments measure air and which measure surfaces, which reference values to use for the most commonly kept species, and which checks to repeat every week so that a drift becomes visible before it turns into a clinical problem.

Why the gradient matters more than the average temperature

The right question is not what temperature to keep the enclosure at, but which range of temperatures to make available. A reptile regulates its own body temperature by moving: it warms up under the heat source when it needs to digest and moves away once it has reached the temperature it needs. If the enclosure has a single temperature, that choice does not exist and the animal stays chronically too warm or too cool, even when the average figure looks correct.

The gradient is built in space, by concentrating the heat source at one end and leaving the other end free. Two practical conditions make it work. The first is length: in a short enclosure the two ends equalise and the gradient disappears, so minimum size is a thermal requirement, not an aesthetic one. The second is structure: branches, rocks and hides must let the animal reach every temperature band without staying exposed, otherwise it will choose safety over the correct temperature and spend the day hidden in the wrong place.

Instruments: what each one actually measures

Most thermal management errors come from confusing air temperature with surface temperature. They are different quantities and are not interchangeable: under a lamp, a rock can sit ten degrees above the air around it.

Instrument, quantity measured and correct use
Instrument	What it measures	Correct use	Limitation to know
Digital probe thermometer with lead	Air temperature at the probe position	Cool end and warm end, at animal height	Reads only the single point where the probe sits
Infrared thermometer	Surface temperature of the area aimed at	Basking spot, floor, rocks and branches	Does not read air and errs on shiny surfaces
Thermostat with probe	Controls the source from one point only	Probe fixed where the safety ceiling matters	If the probe moves, control is lost
Liquid crystal stick on thermometer	Temperature of the glass it is stuck to	No reliable use for gradient work	Wide error and no air reading at all
Digital hygrometer	Relative humidity of the air	Resting area and humid zone	Needs recalibration or replacement over time
Ultraviolet radiometer	Ultraviolet index at the measured point	Checking the lamp and the useful distance	Expensive, often shared among keepers

Calibrating the enclosure in nine steps

Set the enclosure up empty and close it
Arrange furnishings, substrate and hides exactly as in the final layout, but without the animal inside. Every object changes how heat spreads, so calibrating a bare enclosure produces numbers you will never see again.
Place the probes before switching on
Fix the warm end probe where the animal will rest its body, not on the glass and not hanging in mid air. Put the second probe at the opposite end at the same height. Tape the leads down, so an out of range reading will never be caused by a probe that has moved.
Switch on and wait two full hours
An enclosure reaches equilibrium slowly. Reading after ten minutes leads to chasing the number by raising the power and discovering later that you built a dangerous basking spot. Two hours is the minimum, three is better in a large volume.
Measure surfaces with the infrared thermometer
Aim at the basking spot, the floor under the lamp, the nearest rock and the substrate at the cool end. Write down every value. The hottest surface is the one the animal will touch with its belly, and that number decides whether a burn happens.
Adjust height before adjusting power
If the basking spot is too hot, raise the lamp or move the source further away before lowering the power. Cutting the power also shrinks the usable warm area and produces a tiny hot spot the animal cannot fit into.
Set the thermostat as a safety ceiling
The thermostat is not there to pick the ideal temperature, it is there to prevent the ceiling being crossed. Fix its probe at the hottest point the animal can reach and set the threshold at the maximum allowed for the species, never at an average value.
Verify the gradient end to end
Measure air temperature at three aligned points from the warm end to the cool end. You should get a progressive fall. If the three readings are nearly identical, the enclosure is too short or the power is excessive, and neither problem is fixed by a thermostat.
Simulate the night
Switch off the daytime source and measure again after two hours, in the real conditions of the room. This is when you discover that a cold room drags the enclosure below the tolerated minimum. If that happens, the answer is a source without visible light on its own thermostat.
Write the accepted values and repeat the check
Record the final configuration with lamp height, power, thermostat threshold and the readings obtained at each point. Repeat the full measurement one week later and again at every change of season: comparison with these numbers is what makes a drift visible.

Reference values for the most commonly kept species

The ranges below are husbandry references for healthy adults, not prescriptions: an animal under treatment, a gravid female or a specimen in brumation needs different settings, to be agreed with a vet. Surface temperatures are measured with the infrared thermometer, air temperatures with the probe.

Indicative ranges for healthy adults
Species	Basking surface in degrees	Cool end air in degrees	Night minimum in degrees	Relative humidity
Bearded dragon	38 to 42	24 to 26	18 to 22	30 to 40 per cent
Leopard gecko	30 to 33 at floor level	22 to 25	18 to 22	30 to 40 per cent
Crested gecko	24 to 26, never above 28	20 to 22	18 to 22	60 to 80 per cent
Ball python	31 to 33	24 to 26	not below 22	55 to 65 per cent
Corn snake	28 to 30	22 to 25	18 to 22	40 to 60 per cent
Hermann's tortoise indoors	32 to 35	20 to 25	12 to 18	40 to 60 per cent

Temperature goes together with ultraviolet exposure, which is chosen using the Ferguson zones: four bands describing how much a species exposes itself to the sun in the wild and which ultraviolet index matches it at the resting spot.

Zone 1, crepuscular and shade dwelling species: reference ultraviolet index up to 0.7.
Zone 2, species of partly shaded habitats: reference index from 0.7 to 1.0.
Zone 3, species that bask in the sun intermittently: reference index from 1.0 to 2.6.
Zone 4, species that expose themselves to midday sun: reference index from 2.6 to 3.5.
In every zone the animal must be able to leave the irradiated area and enter a fully shaded hide.

The weekly check and the log

The initial calibration does not stay valid forever: lamps lose output, substrate compacts, the room changes temperature with the seasons and a thermostat can fail in the on position. A five minute weekly check catches nearly all of these drifts.

Read and record air temperature at the warm end and the cool end, always at the same hour.
Measure the basking surface with the infrared thermometer and compare it with the value accepted at calibration.
Check that the probes are still in their original position and that the leads have not been moved by the animal.
Note relative humidity along with the last misting or water change.
Record the animal's weight on the same scale, with the date of the last meal and the last shed.
Check the installation date of the ultraviolet lamp: output falls long before the visible light goes out.
Log any intervention, from changing a bulb to moving a branch: without that line a later change stays unexplained.

Common thermal management mistakes

Mistake, effect on the animal and correction
Mistake	Effect	Correction
A stick on thermometer as the only instrument	Unusable readings and an unknown gradient	Two digital probes plus an infrared thermometer
Heat source plugged straight into the socket	No upper limit and risk of ventral burns	Thermostat with the probe at the hottest point
Heating spread along the whole length	No gradient, thermoregulation impossible	Concentrate the source at one end only
Visible light lamp left on at night	Disrupted circadian rhythm and poor rest	A source without light, run by a thermostat
Ultraviolet lamp never replaced	Output has fallen while the light looks unchanged	Scheduled replacement and a recorded date
A hide only at the cool end	The animal chooses cover over correct warmth	One hide in each temperature band
Calibration done in an empty enclosure	Real values with furnishings turn out different	Calibrate in the final layout

Frequently asked questions

My reptile stopped eating: can temperature alone explain it?: It is often the first thing to rule out, but it is not a diagnosis. Below its active range a reptile digests slowly and stops feeding to avoid holding undigested food in the stomach. Before looking for more complex causes, verify gradient, basking spot and night minimum with written readings. If the values have been correct for at least a week and the refusal continues, book a consultation and bring the measurement log with you.
Can I use a heat rock instead of a lamp?: In most situations this is not advisable. Heat rocks create a very hot contact point on a small surface and do not warm the surrounding air, so they do not build a real gradient. Many species perceive heat mainly from above and will not move away from a scorching surface until damage has already occurred. An overhead source under thermostat control remains the more predictable choice.
How often should the ultraviolet lamp be replaced?: Follow the lifespan stated by the manufacturer for that model and write down the installation date on the day you fit it. Ultraviolet output falls long before the lamp stops emitting visible light, so relying on the light itself keeps an ineffective source running for months. If you can borrow a radiometer, a check at the animal's real resting distance is the most reliable control.
Does the temperature really need to fall at night?: For most terrarium species a night time drop of a few degrees reproduces natural conditions and causes no problem, as long as it stays above the minimum for that species. The critical factor is the room: in winter an unheated room can pull the enclosure well below that minimum. Measure for two consecutive nights before deciding, and if needed add a source without visible light on its own thermostat.

What to do next

Calibrate the enclosure in its final layout, with two probes for air and an infrared thermometer for surfaces, and write down the accepted values. Connect every heat source to a thermostat with its probe at the hottest point the animal can reach. Repeat the check weekly, recording temperatures, humidity, weight and lamp date: thermal drift shows up in the comparison between written numbers, never by eye.