UVB and heat for reptiles: why the lamp matters and when to replace it

When a lizard in the wild climbs onto a rock in the morning sun, it's actually doing two things at once: warming up, and using the ultraviolet light hitting its skin to make vitamin D3 in its body. In a terrarium there's no sun; lamps take its place. That's why a reptile's health depends largely on two things: the right UVB lamp and the right heat setup. And just because a UVB lamp is on doesn't mean it's still giving off enough UVB.

Why does UVB matter so much?

According to the Merck Veterinary Manual, UV light is essential for many species to make vitamin D3 in their skin, and a reptile can only absorb the calcium in its food with this vitamin. The RSPCA also says that UVB lets reptiles make the vitamin D3 they need to store and use calcium. According to Merck, the packaging of the right lamp should clearly state that it gives off UVB.

If there isn't enough UVB, the bones weaken. According to the RSPCA, metabolic bone disease, which is common in reptiles kept at home, is often caused by a lack of UVB and can lead to permanent deformities.

Glass, plastic and wire mesh

UVB's biggest enemy is a clear surface in the way. According to the Merck Veterinary Manual, sunlight filtered through glass or plastic has almost no ultraviolet left in it. So there should be no glass or plastic between the lamp and the animal. The RSPCA also says that UVB doesn't pass through window glass, which is why the lamp needs to be placed inside the terrarium.

• A spot by the window isn't UVB: Sunlight behind glass doesn't give your reptile UVB, and a terrarium in direct sunlight can overheat. The RSPCA recommends keeping terrariums out of direct sunlight.
• Wire mesh blocks some of it too: According to the RSPCA, wire mesh blocks part of the UVB; if the lamp sits on top of a mesh lid or the terrarium is tall, a stronger lamp may be needed.
• Distance matters: According to the RSPCA, UVB gets weaker with distance, so place the lamp at the distance the manufacturer recommends.
• Real sunshine: According to Merck, animals that can get outdoors into natural, unfiltered sunlight usually get enough vitamin D3. For bearded dragons, the RSPCA recommends a safe outdoor area with a shaded corner on sunny days; you should never leave your dragon's side while it's out there.

Not every species wants the same lamp

UVB needs vary a lot from species to species. The recommendations on the RSPCA's species pages:

According to the RSPCA, the UV index should drop to zero from the basking spot toward the shade, so the animal can choose for itself how much light it gets. Some color varieties (morphs), such as albinos, need less UVB. For details on aquatic turtles: aquatic turtle care.

Snakes are a little different: according to Merck, full-spectrum (UVB-producing) light is essential for some species, while other species have no special lighting needs; but even for these species, full-spectrum light is still recommended. The RSPCA recommends a low-output UVB tube for corn snakes.

When should the lamp be replaced?

UVB lamps produce less UVB over time, even while they keep giving off light. That's why it isn't enough for the lamp to be on. The RSPCA recommends replacing the lamp at the interval the manufacturer gives, and regularly checking its output with a UV index meter (UVI meter) at the height where the animal sits. The Merck Veterinary Manual recommends replacing fluorescent UVB lamps every 9–12 months. Check the manufacturer's instructions for your own lamp.

• Note the date: Write down the day you fitted the lamp, so you don't forget when it's due for replacement.
• Measure: If you have a UV index meter, measure the light regularly and compare the results.
• The right fitting: The RSPCA recommends fitting the UVB tube with a reflector, in the way the manufacturer recommends.

Heat: a warm side and a cool side

Reptiles can't produce their own body heat. According to the RSPCA, that's why a terrarium needs a heat gradient: a warm basking spot at one end and a cool area at the other. The animal warms up or cools down by moving between the two. The heat lamp and the UVB tube are usually placed at the same, warm end.

1. Thermostat: According to the RSPCA, every heater should be connected to a thermostat.
2. Two thermometers: Thermostats don't always work correctly. Measure the warm and cool ends every day with a digital or infrared thermometer.
3. Night: Depending on the species, the heat is lowered or switched off at night. If the room gets very cold, a ceramic heater that gives off no light can be used.
4. Timer: The RSPCA recommends running the lights on a timer, 12 hours on and 12 hours off; all lights should be off at night.

Protection against burns

A reptile that gets too close to a heat source or touches it can be burned. That's why:

• Protective guard: According to the RSPCA, both heat lamps and UV lamps must be protected with a guard, to prevent burns and injuries if a bulb shatters.
• No "hot rocks": According to Merck, "hot rocks" heated from the inside heat unevenly and can cause burns; they aren't recommended.
• No direct contact: According to Merck, all heat sources should be guarded so the reptile can't touch them directly, and controlled by a thermostat.

Our care articles by species: leopard geckos, corn snakes, bearded dragons, aquatic turtles. All of them: Reptiles.

Frequently asked questions

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Sources

• Merck Veterinary Manual · Providing a home for a reptile
• Merck Veterinary Manual · Management and husbandry of reptiles
• RSPCA · How to care for your bearded dragon
• RSPCA · How to care for a leopard gecko
• RSPCA · How to care for a corn snake

By Tüylüm · Published and last checked: October 24, 2026. Spotted a mistake? Write to [email protected] and we'll fix it right away. Photos are from Wikimedia Commons, used under the license shown below each photo.