What is UVB light?
UVB (315-280 nm) is the spectrum of UV light most reptile and amphibian keepers are concerned with. UVB plays a vital role in the production of Vitamin D3 in your pet reptile or amphibian's body, which facilitates uptake of dietary calcium and fights maladies such as metabolic bone disease. Without UVB, reptiles and amphibians (and you!) cannot efficiently utilize calcium, and will eventually end up deficient in this vital mineral. A lack of calcium results in poor or improper bone growth, and can result in metabolic bone disease. Proper calcium levels are also vitally important to ensure proper function of the nervous system.
While the relationship of UVB, Vitamin D3, and calcium is well known, UVB plays other important physiological roles too. Exposure to healthy levels of UVB light are important for normal functioning of the immune system, cell growth, blood pressure, and even insulin secretion.
Keep in mind that low levels of UVB are present in the shade on a sunny day, and many nocturnal reptiles and amphibians are probably exposed to UVB in nature. Animals that eat whole vertebrates, such as snakes, may not need UVB, as they receive Vitamin D3 from their prey. For most species of reptiles and amphibians, dietary supplementation of Vitamin D3 does NOT provide the same benefits as UVB lighting.