A bearded dragon needs one guarded basking lamp at one end of the enclosure. The RSPCA care sheet suggests a 60–100W bulb, and the dragon’s back should be no closer than 25–30cm to it. The basking surface should read 38–42°C (100–108°F), the cool end 22–26°C (72–79°F), and nights should not drop below 20–22°C (68–72°F). UVB comes from a separate tube. All lights go off at night.
Most guides stop there. This one is about what happens next, when your thermometer or your dragon tells you something is wrong. The core of the guide is a symptom → cause → fix → recheck matrix. Next to it is a table that matches each type of heat bulb to the thermostat it actually works with. A mismatch there causes more broken setups than buying the wrong wattage does.
The numbers to hit (quick reference)
| What you’re measuring | Target | Source |
|---|---|---|
| Basking surface temperature | 38–42°C (100–108°F) | RSPCA |
| Cool end | 22–26°C (72–79°F) | RSPCA |
| Night minimum | Not below 20–22°C (68–72°F) | RSPCA |
| Basking bulb wattage | 60–100W, guarded | RSPCA |
| Lamp to dragon’s back | No closer than 25–30cm (about 10–12 in) | RSPCA |
| UV index at the basking spot | 3.0–5.0, falling to zero in shaded areas | RSPCA |
| Photoperiod | 12 hours on, 12 hours off, on a timer | RSPCA |
| UVB tube replacement | Every 9–12 months (Merck) or every 3–6 months (UC Davis) | Merck, UC Davis |
Heat and UVB do different jobs. Heat lets the dragon regulate its body temperature and digest food. UVB lets it make vitamin D3, which it needs to use calcium. A “full-spectrum” screw-in bulb gives heat but no UVB, as UC Davis notes. Glass blocks UVB completely, and mesh between the tube and the dragon reduces it noticeably. Heat mats and hot rocks are out: Merck warns that they cause belly burns.
Why the numbers don’t match between care sheets
Look up basking temperatures and you’ll find several ranges. The UC Davis care sheet says 88–95°F. The Merck Veterinary Manual gives a preferred zone of 25–32°C and puts the basking spot about 5°C above it. RSPCA says 38–42°C. Hobby sites often go higher still.
Most of the gap comes from what is being measured. The surface of a stone directly under a halogen lamp reads much hotter than the air a few inches above it, and a dragon lying flat on that stone absorbs heat from both. Care sheets were also written at different times, and they don’t always say whether they mean air or surface temperature.
Our rule: measure the basking surface with a probe thermometer taped flat to it, or with an infrared temperature gun, and keep it within RSPCA’s 38–42°C. Treat any reading above that as a fault to fix, not a preference to argue about. Some hobbyist guides set their own danger threshold higher. Those are hobby sources, not veterinary ones, and we don’t use them as a target.
Troubleshooting matrix: symptom → cause → fix → recheck
Find the row that matches what you see. Make one change at a time. Wait for the enclosure to settle (give it the rest of the day), then take the recheck reading before you change anything else.
| Symptom | Likely cause | Fix | Recheck | Call an exotic vet instead when… |
|---|---|---|---|---|
| Basking spot too cold even with the lamp at full power | Cold room; lamp mounted too high; spot vs flood bulb mismatch; thermostat probe reading air instead of surface; a thin basking surface that doesn’t hold heat | Raise the basking platform (never closer than 25–30cm from lamp to the dragon’s back). Switch to a dense, dark stone such as slate. Move the probe to basking level. Warm the room. Only then step up the wattage. See the worked example below. | Basking surface reaches 38–42°C | The dragon has stopped eating or seems weak while you’re still fixing the temperatures |
| Halogen flickering on and off all day | Halogen or incandescent bulb on an on/off thermostat | Swap to a dimming thermostat (see the controller table) | Bulb glows steadily, brighter or dimmer, instead of switching fully on and off. Basking surface holds steady. | Not a health issue on its own |
| Dragon gaping or scratching at the glass | Basking spot too hot; cool end too warm; no gradient (whole tank heated evenly); lamp centred over the tank | Lower the wattage or raise the lamp. Move the lamp to one end. Add ventilation. Remove reflective materials. | Basking surface no higher than 42°C. Cool end 22–26°C. | Gaping continues once temperatures are verified, or you see discharge from the nose or mouth, or laboured breathing |
| Dragon won’t leave the cool end | Basking spot too hot; basking platform unstable or slippery; no hiding place, so the dragon is stressed; the setup is new | Measure the surface. Swap to a stable, textured platform. Add a hide at the cool end. Allow settling time after a move. | Basking surface 38–42°C, and the dragon visits the basking spot during the day | It still won’t bask and isn’t eating several days after the setup passes every check |
| Temperatures look right but the dragon is dark and lethargic | Probe measuring air, so the “right” reading isn’t the surface; old UVB tube; UVB behind glass or mesh; the dragon may be unwell | Check the basking surface with a second thermometer or an IR gun. Meter the UVB or replace the tube. | Surface 38–42°C on an independent reading. UVI 3–5 at basking level. | The dragon stays dark and lethargic once you’ve confirmed the surface temperature and UVB. Book a vet visit; don’t keep adjusting. |
| Nights below 20°C | Cold room; no night heat source | Add a ceramic heat emitter (heat, no light) on its own thermostat. Do not leave the basking lamp on, and do not use red or blue “night” bulbs. | Night reading at dragon level not below 20–22°C | The dragon is sluggish and off food during cold spells even after the fix |
| UVI below 3 at the basking spot | Tube past its replacement date; tube mounted too high; mesh or glass in the way; screw-in “full-spectrum” bulb instead of a real UVB tube | Replace the tube and write the date on the fixture. Lower it, or mount it inside the enclosure, following the manufacturer’s distance guidance. Remove any glass in the way. | UVI 3.0–5.0 at basking level, falling to zero in the shade | You notice a soft or rubbery jaw, trouble walking or climbing, swollen limbs, or tremors |
| Heat bulb burns out early | Rapid on/off cycling from the wrong thermostat; socket not rated for the wattage; loose bulb or a cheap power strip | Use a dimming thermostat for halogens. Fit a ceramic socket rated above your bulb’s wattage. Plug into the wall or into a quality surge strip. | Bulb outlasts its predecessor, and the socket and cord don’t get hot to the touch | Not a health issue on its own. If the fixture shows scorching, unplug it. |
| Whole tank heats evenly / cool end too warm | Too much wattage for the enclosure; glass lid trapping heat; warm room; lamp in the middle | Drop wattage. Switch to a mesh top or add vents. Move the lamp to one end. Cool the room in summer. | Cool end 22–26°C while the basking spot stays 38–42°C | The dragon shows overheating signs (gaping, digging, frantic glass-scratching) that don’t settle once the cool end is cool |
Do I need a dimming thermostat or an on/off thermostat?
It depends on the bulb, and getting this wrong is behind several rows in the matrix.
| Heat source | Controller | Why |
|---|---|---|
| Halogen flood or incandescent basking bulb | Dimming thermostat | An on/off thermostat cuts the bulb to zero and back again all day. The basking spot swings, the light flickers, and bulbs fail sooner. A dimmer adjusts power smoothly. |
| Ceramic heat emitter | On/off or pulse-proportional thermostat | It gives no light, so cycling doesn’t affect the photoperiod. A pulse thermostat holds temperature more tightly. |
| Mercury vapour bulb (heat + UVB) | No dimmer. Adjust by height. | These bulbs generally aren’t designed to run on a dimmer. Check the manufacturer’s instructions for your specific bulb. Get the temperature right by changing the mounting height, and confirm with a UVI meter. |
| Heat mat / hot rock | Don’t use | Merck warns about belly burns |
Where does the thermostat probe go?
Sources disagree. RSPCA puts the probe at the height where the dragon sits. Some hobbyist guides put it in the cool zone as a safety cutoff. Here is what works in practice:
- Control probe at basking level, taped flat to the basking surface or right beside it, where the dragon can’t lie on it or knock it off. A probe dangling in the air reads cooler than the surface. The thermostat then drives the lamp harder, and the stone ends up hotter than your display says.
- Check it independently. Keep a separate digital thermometer or IR gun. Your thermostat display tells you what the probe senses, not what the dragon is lying on.
- Test that the thermostat is actually regulating. Warm the probe in your hand for a few minutes. A dimming thermostat should visibly dim the lamp. If nothing changes, it isn’t controlling anything.
What wattage heat bulb do I need for a 40, 75 or 120 gallon tank?
Tank volume is a poor predictor. What decides it is your room temperature, how high the lamp sits, whether the enclosure has a mesh or solid top, and whether you use a flood or spot bulb. RSPCA’s 60–100W range covers most setups. Use it like this:
- 40 gallon: Start at the low end of the range. In a tank this size, the bigger risk is heating the whole space so the cool end never gets cool. If the cool end creeps above 26°C, the wattage is too high.
- 75 gallon: Start in the middle of the range and adjust from what you measure.
- 120 gallon (4×2×2 ft): Start at the top of the range. The extra height and floor space lose more heat, especially in a cool room. If 100W can’t reach the target, work through the worked example below before buying a bigger bulb.
Is 150W too much? Not automatically, but it is above the range the RSPCA sheet covers. At the minimum safe distance, a higher-wattage bulb makes a hotter, more concentrated spot that can overshoot. If you go above 100W, run it on a dimming thermostat. Keep the 25–30cm distance to the dragon’s back, and confirm the surface never goes above 42°C.
How far should the heat lamp be from the basking spot?
RSPCA’s rule is about the dragon’s back, not the platform: no closer than 25–30cm. Measure from the bulb face down to the platform, then subtract the height of your dragon lying flat. An adult adds several centimetres. The lamp also needs a guard, so the dragon can’t climb up to the bulb.
Once the distance is set, adjust temperature by changing wattage or the thermostat setting. Don’t adjust it by moving the dragon closer.
Should the heat lamp stay on at night, and what should I use for night heat?
No. All lights go off at night. That includes red and blue “night” bulbs, which still emit light the dragon can see. If your room keeps the enclosure at 20–22°C or above overnight, you don’t need night heat at all.
If it drops lower, use a ceramic heat emitter on its own thermostat, mounted in a guarded fixture. Choose the lowest wattage that holds 20–22°C on your coldest night. Start low and step up, because an oversized emitter can make nights warmer than days.
Can I use a regular household halogen bulb instead of a reptile basking bulb?
For heat, yes, with conditions. A household halogen flood bulb in RSPCA’s wattage range heats a basking spot the same way a reptile-branded basking bulb does. Use one with a wide beam, not a narrow spot. Put it in a ceramic socket rated for the wattage and run it on a dimming thermostat.
What no household bulb does is replace UVB. UC Davis specifically notes that screw-in “full-spectrum” bulbs provide heat but not UVB. You still need a real UVB tube.
My 100W bulb won’t get the basking spot above 100°F. What should I change?
Here is a worked example of the order to try fixes in.
Situation: An adult dragon in a 4×2×2 ft (120 gallon) enclosure with a mesh top, in a basement room that sits around 64°F (18°C). A 100W halogen flood is on a dimming thermostat set to 104°F (40°C), mounted 40cm above a thin ceramic tile. The dragon is about 6cm tall lying flat, so lamp-to-back distance is about 34cm.
- Check what’s being measured. The thermostat display reads 104°F, but an IR gun on the tile reads 95°F. The probe was clipped to a branch above the tile. Tape the probe flat beside the basking spot. The thermostat now sees the real surface and runs the bulb at full power, but the surface still only reaches 97°F. Fail.
- Raise the platform, not the risk. Add a 4cm riser under the basking spot. Lamp-to-back distance drops from about 34cm to about 30cm, which is still within RSPCA’s minimum. Surface reading: 99°F. Still short.
- Swap the surface. Replace the thin tile with a thick, dark slate slab, which absorbs and holds heat better. Recheck the next day after the morning warm-up: 102°F (about 39°C). Pass.
- Check the rest of the gradient. Cool end: 73°F (about 23°C). Pass. Overnight at dragon level: 66°F (about 19°C). Fail, because it’s below 20°C.
- Fix the night. Add a low-wattage ceramic heat emitter on its own on/off thermostat, set to 70°F (21°C) with the probe at dragon level. Next morning’s minimum reading: 69°F. Pass.
Changing the wattage was never needed. If step 3 had still fallen short, the next moves would be a second matching flood bulb beside the first (to widen the spot rather than make it hotter) or warming the room. A single hotter bulb pushed close is the last option, not the first.
Commissioning loop: run it for a week before the dragon arrives
RSPCA advises running the enclosure for at least a week before the dragon moves in. Treat that week as a test with pass/fail limits, not just a waiting period.
Daily readings. Take them at the same times each day: after the morning warm-up, at midday, before lights-off, and overnight.
| Check | Pass | If it fails |
|---|---|---|
| Basking surface (independent thermometer or IR gun) | 38–42°C (100–108°F) | Use the “too cold” or “gaping” rows of the matrix |
| Thermostat display vs independent reading | They agree closely | Move the probe to basking level and retest |
| Cool end floor | 22–26°C (72–79°F) | Use the “whole tank heats evenly” row |
| Night minimum at dragon level | Not below 20–22°C | Add or adjust the ceramic emitter |
| UVI at basking level | 3.0–5.0, falling toward zero in shade | Use the “UVI below 3” row |
| Halogen behaviour | Steady glow, no flicker | Swap to a dimming thermostat |
| Timer | Lights on and off on a 12-hour schedule | Reset the timer |
| Fixtures and cords | Guarded, secure, not hot to the touch | Re-mount or replace |
The loop: if any check fails, fix it using the matrix, then restart the count. The dragon moves in after a full week in which every check passes.
Common mistakes
| Mistake | What you’ll see | Fix |
|---|---|---|
| Probe in the air, not at basking level | Display looks right, but the surface is too hot or too cold | Tape the probe at basking level and verify independently |
| Halogen on an on/off thermostat | Flicker, swinging temperatures, early burnouts | Use a dimming thermostat |
| Basking lamp left on overnight, or a red “night” bulb | Disrupted day/night cycle | Turn all lights off at night. Use a ceramic emitter if you need heat. |
| UVB tube behind glass or under dense mesh | Low UVI reading despite a “new” tube | Move the tube above open mesh or inside the enclosure |
| UVB tube past its replacement date | Tube looks bright but the meter reads low | Replace it on schedule and write the date on the fixture |
| Lamp centred over the tank | No gradient | Move the lamp to one end |
| Platform pushed too close to the bulb | Avoids the spot, or marks on the back | Restore the 25–30cm lamp-to-back distance |
| Stick-on dial thermometer | Readings that don’t match how the dragon behaves | Use a digital probe thermometer or IR gun |
| Heat mat or hot rock | Belly burns | Remove it and heat from above |
The kit
- Basking bulb: a halogen flood in the 60–100W range
- Deep-dome fixture with a ceramic socket rated above the bulb’s wattage, plus a lamp guard
- Dimming thermostat with probe
- Ceramic heat emitter, its own thermostat, and a guarded fixture (only if nights fall below 20–22°C)
- Digital probe thermometer and/or IR temperature gun
- Thick, flat basking stone such as slate
- Linear UVB tube and fixture, separate from the heat lamp
- Plug-in timer
- UVI meter (optional, but it’s the only way to know your UVB is still working)
When to stop adjusting and see an exotic vet
Fixing the setup is your job. Diagnosing the dragon isn’t. Once your checks pass and the dragon still shows any of the following, book an appointment with a reptile vet. Look for an exotic-animal vet rather than a general practice, and ask whether they see bearded dragons regularly.
- Soft or rubbery jaw, trouble walking or climbing, swollen limbs, or visible bone changes
- Muscle twitching, tremors, or seizures
- Burns, blisters, or discoloured patches of skin
- Discharge from the nose or mouth, or laboured, open-mouth breathing once temperatures are confirmed normal
- Refusing food, or staying dark and lethargic, despite verified surface temperatures and UVB
- No droppings for longer than is normal for your dragon, especially with a swollen belly or straining
For the science behind UVB for captive reptiles, including why distance and bulb choice matter as much as the bulb itself, LafeberVet’s evidence-based update on UVB lighting is written for veterinary professionals and worth reading. For UVB tube choice and placement in more depth, see our lighting requirements guide.