Heat and UVB: What Each One Actually Does
Heat and UVB get discussed together and do completely different jobs. Confusing them is behind a large share of the husbandry failures we write about.
They are not interchangeable
Heat lets a reptile digest, move and fight infection. UVB lets most diurnal species synthesise vitamin D3, which is what allows dietary calcium to be used at all. A perfectly heated enclosure with no UVB can still produce metabolic bone disease in a species that needs it, and a UVB bulb that produces no meaningful heat will not fix a cold animal.
Thermostats are not optional
A heat mat or ceramic emitter plugged straight into the wall has no upper limit. Reptiles are poor at detecting ventral heat and will sit on a surface that is burning them. A thermostat with the probe secured at the contact surface is the difference between a working setup and a vet visit.
Measuring temperature where it matters
The number that counts is the surface the animal rests on, taken with an infrared temperature gun or a contact probe. A dial thermometer stuck to the glass reports air temperature somewhere irrelevant. Check the basking surface, the cool end, and the floor of the hide.
UVB fades invisibly
A fluorescent UVB tube keeps emitting visible light long after its useful UVB output has dropped away. That is why replacement is scheduled by date rather than by failure, on whatever interval the manufacturer specifies. Distance and any mesh between bulb and animal both cut output further, so mounting height is part of the dose, not a detail.
Common questions
Do all reptiles need UVB?
No, but more need it than the old advice suggested. Diurnal baskers clearly do. Several nocturnal and crepuscular species that were long kept without it appear to benefit from low-level UVB. Because the answer is species-specific and has genuinely changed with research, check the species page rather than a general rule.
Can a heat lamp replace a UVB bulb?
No. Unless a bulb is specifically a UVB-emitting product, heat and light output tell you nothing about UVB. A bright, hot enclosure can still be a UVB-free one, and that is a common route to metabolic bone disease.
Do I need a thermostat for a heat mat?
Yes. An unregulated mat has no ceiling, and reptiles are poor at sensing heat from below, so an animal may stay on a surface hot enough to burn it. The probe should sit at the surface the animal contacts.
How often should I replace a UVB bulb?
On the manufacturer's stated schedule, not when it stops lighting. UVB output decays well ahead of visible failure, so a bulb that looks fine can be delivering very little. Write the install date on the bulb itself.
Sources
Related reading
all husbandry fundamentals · common mistakes · finding a reptile vet