You've just walked into your camping spot and have just used X amount of energy to get into where you are going. Cooked dinner and eaten to replaced that lost fuel. Now it's bed time so getting a good night's sleep is really important.
The whole point of a sleeping mat is to insulate you from the cold ground. If you don't insulate yourself from the cold ground a few things can happen 1/ you will just get colder and colder. 2/ the cold ground will suck all you body warmth away 3/ you will just not sleep but shiver and shiver uncontrollably which will result in near zero sleep and without good warmth and sleep you run the risk, next day of having an accident or doing something silly, possibly even hypothermia because your already chilled (depending on weather conditions).\
In cold conditions and freezing ground an insulated sleeping mat is essential.
Once again there are a lot of different types about, a closed foam rolled mat will work perfectly well and you could pick one up from a second hand store for $5. That is all you need at this stage. But let's say your loaded and have lots of money and decide to buy the latest insulated Thermarest for around $600. Again, in my opinion, money well spent. They are about 90% smaller than the closed foam rolled mat but when inflated are thicker, give better insulation, are more comfortable and they take up a very small amount of space in your pack. And deliver a better night's sleep.
A five dollar closed foam mat like this is all you need to get you started.
People think that by buying a half sized sleeping mat they are saving room and weight and that only the top half of you needs insulating. Some people do this and use their pack to lie their legs on to insulate that lower area. In my opinion, it is easier to just buy a full length sleeping mat that covers your whole body and it is now something you don't have to think about or worry about. Fully covered is better than half covered.
My Thermarest sleeping mat, compact and light weight
Again just like packs, tents, sleeping bags etc, do your homework, shop around, ask other friends or back country users what are they using. As I've mentioned above you don't have to go out and buy the latest Thermarest at $700 that you are only going to use 4-5 time a year maybe. That $5 closed foam bed roll, might be all you need at this stage.
The inflated Thermarest Neo-air. A great bit of kit, but at around $700 getting up there in price.
Over the years I've used lots of different types of sleeping mats and I now use one of the Thermarest Neo-air inflating air mattress , they are awesome. If you are going to go camping, sleeping in a tent on the ground you really do need to have a sleeping mat. Whether you have the $5 or the $600 one doesn't matter, the important thing is that you have a insulated sleeping mat and you are insulated from the cold ground.