If the enclosure can be positioned where it receives some natural sunlight, without exposing it to draughts or excessive heat, so much the better. But sunlight looses much of its spectrum when passed through glass and because light of a suitable spectrum and intensity is essential to the lizards' well-being it is necessary to invest in a lighting system that imitates the spectrum of sunlight The cheapest are fluorescent tubes that do not generate heat and run at low wattages. A full spectrum tube such as "Vitalite" provides all necessary wavelengths including ultra-violet which is essential for many lizards to synthesize vitamin D3, Ultra·violet radiation can be injurious to the eyes and lights are only effective when the lizard and bulb are in close proximity. You should check this with the manufacturer. Not everyone is convinced that monitor lizards need a source of ultra-violet light if they are supplied with adequate D3 as a dietary supplement. Other lamps reproduce the sun's spectrum without producing ultra-violet light. More expensive options are mercury vapour lamps which give a very faithful rendition of natural sunlight and generate heat as well. Tungsten light bulbs and white fluorescent tubes are very poor substitutes. A wide variety of light sources available and prices vary considerably, so it is wise to ask around (especially at your local herpetological society meetings) before making any expensive commitments

Providing light of the correct intensity is as important as providing a natural spectrum. The intensity of light experienced by monitor lizards in the wild varies between species; the large eyes of many rainforest dwellers suggests that they live in relative gloom compared to those exposed to the harsh desert sun. Whilst there is no doubt that changes in photoperiod are important for many monitor lizards there is, as yet, no such evidence that changes in light intensity are of any consequence.

Seasonal Variations in Day Length
In many animals (and plants) changes in daylength are responsible for triggering a variety of responses including initiation of changes in physiology related to breeding and hibernation. Even around the equator, where daylength only varies by a few minutes throughout the year, changes in photoperiod may have a major influence on the lizards' behaviour and physiologies. For this reason it is a very good idea to expose captive animals to proper photoperiods. This is easily done with a simple timeswitch that can be progranuned to go on in the morning and off in the evening. More expensive devices also allow a period of dawn and dusk that maybe beneficial to some species. By altering the photoperiod by a few minutes each week a natural cycle can be followed. Ideally the cycle used should follow the seasons as experienced in the animals' home country rather than locally. With animals from another hemisphere however this often proves inconvenient because the lights will be on longer in the winter than in the summer. Best success has been obtained by maintaining natural cycles but many people believe that if young animals are obtained they should have no problems adapting to a reversal in light cycles. Seasonal changes in light intensity can also be replicated by the use of additional lamps or bulbs of a higher wattage. Data on daylength and elevation of the sun in different parts of the world are given by Jones (1978). Although the term daylength is used here, the important factor is often the duration of the hours of darkness. therefore at night the lizards should be shielded from peripheral light sources. There is no evidence that monitor lizards are influenced by lunar cycles to any great extent.

Attribution / Courtesy: Daniel Bennett. 1995. A Little Book of Monitor Lizards. Viper Press U.K.