Many species of reptiles, especially reptiles active during the day, benefit greatly from a basking light in captivity. Reptiles are ectothermic - they depend on their environment to maintain a regular body temperature. Basking lights, positioned above the enclosure, mimic the heat produced by the sun. Most basking bulbs do not produce needed UVB light, but there are some exceptions (such as Mercury Vapor or Metal Halide bulbs). For the sake of this article, we will exclude red heat bulbs and moonlight bulbs, as they are typically used as an ambient heat source and not to provide a basking spot.
Basking bulbs are generally used about 12 hours a day (10 in the winter, 14 in the summer, if you're cycling your pets to mimic their natural seasonal cycle). Each species of reptiles has an ideal body temperature it needs to be able to reach - select the proper basking bulb that will provide optimal basking temperatures. Position the basking bulb on one side of the tank - this will increase temperatures at one end of the tank and allow the other end to be cooler, providing your pet reptile with a thermal gradient so that it can thermoregulate (move back and forth to regulate it's internal body temperature
Maintaining recommended body temperature is vital to your pet reptile's well being. Each species of reptile has evolved to live at an ideal temperature range, which allows proper digestion of food and utilization of nutrients, activity level, immune system function, natural behaviors, etc. Reptiles that are not able to maintain a recommended body temperature may exhibit a lack of appetite, reduced activity, and eventually die.