Most tortoises (with the exception of some rainforest species) are used to almost constant bright sunshine in the wild and are unable to perform basic functions such as digesting their food, reproducing and even moving about without sunlight or some form of artificial sunlight. The sun provides tortoises and turtles with two things, heat and light. As all tortoises are reptiles this means that they are cold blooded (poikilothermic) and can therefore not regulate their own body temperature, but must rely on the climate in which they live and its fluctuations and microclimates in order to maintain and adjust their body temperature. In the wild all of this heat comes from the natural sunshine, and this is the reason why tortoises are not found in the cooler climatic zones of the world.

The second thing which the sun does for tortoises is to provide ultra violet light in the form of UVa and UVb rays. In captivity this light has to be provided artificially for your tortoise and especially if your tortoise is to be kept indoors for a significant part of the year. Tortoises need UVa light in order to maintain their general behaviour patterns and to remain active and healthy. For instance, tortoises which get plenty of UVa light are reputed to be more likely to breed successfully.

The main requirement of your tortoise is UVb light. Tortoises need a large amount of calcium in their diet in order to grow normally and produce healthy bones and shells. In order to metabolise this calcium your tortoise also needs adequate supplies of vitamin D3. In the wild, tortoises produce their own vitamin D3 from the UVb rays in the sunlight. In captivity it is vital therefore, if you do not want your tortoise to become seriously ill, that you provide adequate amounts of UVb light for your tortoise. In summer this is relatively easy if your tortoise is put outside in the sunshine. When your tortoise is indoors, artificial light is required and this is best provided by special fluorescent tubes (note:normal fluorescent tubes will not do) available from specialist pet stores. The normal UVb tubes (often rated 5.0) are sufficient and very high rated tubes may be detrimental to your tortoises' sight in the long term. The larger the tubes and the more you have the better for your tortoise, however it is advisable to have these tubes at one side of the enclosure (away from the hides/shelters and at the same end as the basking lamps is best) to provide a light gradient within the enclosure. Fluorescent tubes need to be situated as close to the tortoise as possible to provide adequate UVb light. If the tubes are more than 18 inches away from the tortoises then much of their effect will be lost. Fluorescent tubes give out a negligible amount of heat and can not be used as a substitute for basking lamps. Fluorescent tubes must be changed at least every 12 months as they lose their effectiveness over time and stop emitting UVb rays.

Other forms of light which emit UVb rays are now available such as mercury vapour bulbs. These bulbs emit UVB and heat and can be used as light and basking heat sources. These sort of bulbs are usually much more expensive than the equivalent fluorescent, but they can be useful if space is limited and they emit effective levels of UVb rays for a much longer period than fluorescent tubes, which means that they need changing less frequently.

It is very important that the lights you buy for your tortoise are designed for reptile use and state clearly on the packaging that they produce UVb rays. Many lamps and bulbs which state that they are 'full spectrum' or 'wide spectrum' do produce a variety of light wavelengths, but may produce a negligible amount of UVa and UVb rays if any! Also be aware that there is no industry standard for grading bulbs UVb emmision using percentages and that therefore, bulbs strength and effectiveness may vary from manufacturer to manufacturer.