The placement of your heat bulb depends on multiple things, the enclosure size and the species of reptile you are keeping. Regardless of the species, all reptiles are required to thermoregulate, so they all require a temperature gradient within the enclosure. This is done by placing the heat source on one side/end of the enclosure, leaving the other side without heat to create an area that is cooler than the side with the direct heat source. This allows your reptile to thermoregulate by residing to either side it feels it requires. Some reptiles also need UVB bulbs, so for this reason, placing your heat source on one side of the enclosure still gives you room to install a UVB bulb on the other side.