Sleeping mats are designed to insulate you from the ground and at the same time to provide an element of comfort.
Ground Insulation
Although a sleep mat will make the ground a bit more comfortable to sleep on, softening any bumps and irregularities underneath you, this is not its main purpose.
Most of your heat loss during the night is to the ground through conduction. Although your sleeping bag will reduce this, it is not very effective because your weight flattens the bag reducing its insulating properties. This is overcome by using a sleep mat.
Even when sleeping off the ground on a metal framed camp bed or in a hammock, a sleep mat is still a very important part of your camping equipment, and we are sure you will find something here to suit your needs.