The key factors to take in to consideration when choosing a sleeping mat are the pack size, weight, your budget and most importantly what time of year you will be using it which, determines the level of insulation you need. Comfort in an outdoor sense is partly dictated by being able to iron out the lumps and bumps in the ground with the mat, but insulation is king and if you’re cold you won’t sleep. The insulation value of a sleeping mat can be measured and is called the “R-Value”. An international standard recognised by the majority of brands allows for direct comparison, with a full explanation of 'R-Value' can be found here.
Clearly your mat is only part of your sleep system with your sleeping bag, or blanket comprising the other half (not forgetting your pillow). Sleeping bags will only perform at the temperature they have been recorded at provided they are coupled with a warm enough mat. The industry standard for measuring the temperature rating of a sleeping bag uses a sleeping mat with an R-Value of 4.9 which, is considered a winter rated mat. A more insulative mat needn’t be heavier, or bulkier, and there’s no downside come the warmer months, unlike using a sleeping bag that is too warm for the conditions.
Once you have thought about the time of year you are going to use your mat most of all and therefore, the amount of insulation you might need, there are three main categories of mat to consider. These are closed cell foam mats, self-inflating mats and ultralight airbeds. This is where you will be able to focus your other search criteria, such as pack size, weights and cost.
These were the original camping mats; they consist of a thin piece of foam that insulates the sleeper from the ground. They provide some comfort but not very much because they aren't very thick and therefore your body has nothing to sink into. However, they are very light and durable. They can't be punctured - you can cut them to the size and shape you want to shave further weight, and are relatively cheap.
Some closed cell foam mats have been designed with ridges and bumps in order to improve their comfort, and reduce their pack size. This works, but only to a degree. The insulation value of closed cell mats tends to be equivalent to the thinnest self-inflating mats making them only suitable for use in the warmer seasons when used alone. However, they are ideal for boosting the insulation of a sleep system by layering them under another mat in the colder months. Due to their size they will almost always have to be strapped on the outside of your pack. Should they get wet you can always place them under the groundsheet of your tent, where as well as you benefiting from the insulation, it will also protect your groundsheet. Closed cell foam mats have the largest pack size of the three main types of mat.
These mats consist of 'open cell' foam sandwiched between two sheets of material. If the mat is stored inflated between trips the foam retains its “memory” and when packed and subsequently unpacked the foam will expand drawing in air. You will still have to add air, but not nearly as much as when filling an ultralight airbed.
These mats are more comfortable and provide more insulation than a closed cell foam mat. They can get punctures but will still provide some cushioning and insulation due to the foam inside - unlike airbeds. They tend to be heavier than air inflatable mats and closed cell foam mats.
The original airbeds used to be very heavy and were cold to sleep on as the air circulated inside the mat taking heat away from the body. Modern examples are remarkable feats of engineering that now provide good insulation by keeping the air trapped in pockets and, with some mats, by adding insulation, or heat reflective layers. They are made from highly engineered materials that are both light and durable. Often, air mats are supplied with some form of pump sack for inflation and there’s also the tiny battery operated pumps available, or you can resort to good old-fashioned breath to save the most weight.
Depending on their construction, nothing can beat this type of mat for their warmth to weight ratio, and with the standard thickness usually twice that of a self-inflating mat they’re generally more comfortable for that very reason. For the user wanting something light, warm and compact they have become the standard choice. One slight disadvantage of this type of mat is that if it gets a puncture you lose both comfort and insulation of your mat - at least with a self-inflating mat you have the backup of the foam for insulation. Air inflatable mats, more often than not, will be packaged with a small repair kit for field puncture repairs.
When it comes to size, 'Regular' or full-length mats are normally about six-foot-long (183cm), these are the most popular as they fit most people. The majority of mats are “mummy” shaped, but rectangular versions are sometimes available for a larger sleeping area. Inside a tent these fill out more of the floor space and don’t move around as much either. Longer mats are often wider too, which provide more comfort for those who move around in their sleep, or simply if you’re taller. Some manufacturers also make slightly shorter mats for smaller people or women's specific mats which tend to be shorter than a Men's 'Regular'. Women’s specific mats are generally warmer when compared directly to the men’s equivalent as women sleep colder, so these are worth consideration for men too (if you fit).
Short mats, sometimes called 'Torso' mats are very short and are intended to support the upper part of the body only. People buy these because they are lighter than larger mats and they are happy to sleep with their legs unsupported. Often users of torso mats will put their rucksack under their legs for a bit of insulation from the ground, and to bring the legs up to the same height as the mat. For winter season usage the weight saving versus loss in insulation is hard to justify, so torso mats are more suited to the warmer months.