Sleeping when on the trail is very important and lack of it can make easy days very challenging. Having a comfortable night’s sleep comes down to a large number of factors which can’t be controlled properly with only one piece of gear. There are four key items to every sleeping system, all of which play slightly different roles in keeping you warm, dry and comfortable.
What Keeps You Warm?
When sleeping you generate significantly less heat than you do when exercising. This means you are much more susceptible to getting cold when resting than at other times. A common misconception is that “warm” clothing and sleeping bags will warm you up. Really, they actually work to stop your temperature from changing. Therefore staying warm at night is more about preventing heat transfer than actually warming up. Different materials transfer heat at different rates and the important ones for camping are as follows:
Interestingly down, synthetic and foam materials themselves transfer heat faster than still air. Hence their use in insulation systems such as sleeping bags and mats is primarily to stop air from moving rather than to provide insulation value themselves. The best material at doing this is foam, which is why there are so many sleeping mats out there that are made exclusively of foam such as the Thermarest Z-lite series. These mats can’t be compressed very well though and hence synthetic and down mats are on the market. The synthetic fibers used to fill mats and bags are significantly cheaper to manufacture than down and when it gets wet it maintains a lot more warmth than down does. Despite this, down is often the preferred fill for insulation purposes as in ordinary conditions it is lighter, warmer and more compressible.
Ground insulation is arguably the most important part of the sleeping system in terms of warmth and comfort, especially when you are inside a tent. When inside a tent or bivy the air is relatively still and hence you will be losing most of your heat to the ground. On top of this, using a mat makes the surface significantly more comfortable to sleep on.
R-values are often used to compare the insulating properties of mattresses, but in reality R-values can be assigned to just about any insulating system. They typically range between 1.5 (poor insulation) and 9 (good insulation) and essentially indicates the ratio between the temperature difference (between your body and the environment) and the rate at which heat flows.
For camping purposes, it is most effectively used as a method of comparison. In this case being ‘too warm’ is less of an issue than in other parts of the sleeping system.
Sleeping Mat Materials
Foam mats tend to provide the best R-value relative to their thickness due to their superior air-trapping properties. Despite this they lack the comfort factor of inflatable mats and also cannot be compressed for packing. So, while they are the best insulator of the three styles, they are not practical for achieving high R-values (which require larger thicknesses). Foam mats are incredibly robust as they do not suffer from punctures and are cheaper than inflatables. Hence, they are often carried as a backup by mountaineers who depend absolutely on their mats for survival and school groups where mats are often treated with a lower level of care.
Inflatable mats that contain a fill are a lot more comfortable than foam mats, and they can be inflated such that they are thicker and they pack smaller. This means that they can generate much higher R-values without introducing too much bulk. They are the choice of most hikers and campers for these reasons. However, they are quite vulnerable to puncture and so it is a good idea to carry a small repair kit. These repair kits can be purchased separately but also come as part of the package in ranges such as Exped mats. At the upper end of the warmth scale, the mats can become heavy due to more fill being needed, and so down is often used as it has a better warmth to weight ratio than synthetic fibers. Down mats are often used as a primary mat by mountaineers for their weight and warmth.
Other Sleeping Mat Considerations
Other features that distinguish inflatable mats are their face fabrics and their method of inflation. Many mats come in a standard model and a lightweight model. Whilst it can often be tempting to simply go with the lighter of the two, the standard models often offer noticeably more durability, make less noise when moved on and need less care with where they are placed. The lighter models do excel in applications where they must be carried for long distances though. The three main methods of inflation are self-inflating, pump-inflating and bag-inflating. Self-inflating mats take a lot less effort to inflate but takes longer to do so; they are also bulkier due to the memory foam in their construction and hence tend to be thinner. Pump-inflating models are slow to inflate and take a lot of effort. They also require that you bring a pump with you. Whilst they require slightly more effort to inflate than self-inflating models, bag-inflating models are by far the most efficient, as the bag can also then be used as a drybag.
Sleeping Bags are what most people consider first when preparing a sleeping system as they can provide substantial amounts of insulation especially when inside a tent. It is important that the correct sleeping bag for the application is chosen though as they can have major effects on comfort, both positive and negative. They come in a range of different shapes and two different fills which greatly affect their efficiency in both insulation and packing.
Sleeping Bag Fill Materials
The two main fill materials that are used in sleeping bags are down and synthetic. Synthetic fills are easier to launder, are cheaper and maintain more of their warmth when wet than down does. This makes synthetic bags especially good for school groups and travelers who are often less particular with how they treat their sleeping bags. Down bags have a far superior warmth to weight ratio and tend to pack smaller than synthetic bags. This makes them the standard for both hiking and mountaineering where they are to be carried. While down fill bags are more expensive, careful treatment will allow them to last just as long as a synthetic bag would. In high quality sleeping bags the down often receives water resistance treatments to help mitigate issues with the fill getting wet to a certain extent.
Down is generally given a loft rating (or ‘fill power’) which is simply a measure of its quality. The way this rating is calculated is by taking an ounce (roughly 30 grams) of down feathers and then letting it maximally expand. The loft rating is then the volume that the down expands to take up in cubic inches. The ratings tend to vary from around 400 at the lower end up to 800+ for very high quality down. A higher loft rating means more trapped air per ounce of down and hence the down is warmer for the same weight of down. This means you can either make an equally warm sleeping bag lighter or you can make an equally weighted sleeping bag warmer by using higher loft rated down. It is important when determining the warmth of a bag to take into account the weight of the down as well as the loft as the loft rating is a measure of efficiency rather than warmth. For example a sleeping bag with 400 grams of 850 loft down will be less insulating than a bag with 1000 grams of 600 loft down. Because of this rating system, it is not important to differentiate between duck and goose down. When choosing a sleeping bag it is a good idea to compare these values to ensure you are getting an optimal warmth to weight ratio.
Sleeping Bag Shapes and Structure
There are three main shapes that sleeping bags come in; rectangular, tapered and mummy. The closer the shape is to the shape of a body, the more efficient it is at insulating. This is because tighter fitting shapes have less air pockets where there is moving air. Having a close fitting shape such as the mummy shape can feel restrictive though, and so for sleeping bags designed for warmer climates or travel a rectangular or tapered shape is often preferred to the mummy profile. Often the rectangular profile doesn’t have a hood. Whilst allowing more movement within the bag these shapes also allow for the bag to be opened up and used as a quilt more easily. Whilst the mummy shape is usually the pick for mountaineers, hikers tend to prefer the compromise offered by a tapered shape with a purely rectangular bag usually being reserved for travel applications.
The structure of the baffles in sleeping bags also makes a difference as to their warmth. Baffles are traditionally aligned perpendicular to the body . They act to isolate the down into compartments in an effort to maintain an even distribution of fill. Because of their shape, they allow the down to move along their length. When placed horizontally this can mean that the down follows a tendency to fall away from the center of the body and to the sides. This can leave the core with cold patches and hinder its insulation. To combat this modern bags often have the chest baffles aligned vertically. Smaller baffles on the inside of zippers and around the collar of the bag also help to prevent heat loss through cold spots. Higher-end bags also tend to use the box baffle system which helps to maintain an even down coverage over the body with no thin spots where baffles meet, removing the impact of cold spots associated with traditional "stitch through" baffles.
Sleeping Bag Fabrics
The fabrics used on sleeping bags are chosen for a number of reasons but their most important feature is breathability. A more breathable fabric will mean less moisture buildup on the inside of the bag which could affect the performance of the down. These fabrics also often have a level of water resistance to further protect the bag. Durability, compressibility and weight are also considerations. It is important to remember that the lighter the face fabric, the more care you must take when using the bag to ensure its longevity.
Other Common Features
Other features that often distinguish high quality sleeping bags are how they are filled, whether they have pockets for heat packs and whether the zippers are protected from snags. These bags often distribute the down unevenly between the top and bottom of the bag. Because the bottom of the bag becomes compressed under body weight there is less air trapped to insulate the person. This means it is less effective to have down under the body than it is to have it on top of the body. Hence higher end bags will often have more down in the top of the bag than the bottom.
Sleeping Bag Liners
Liners have two main roles in the sleeping system. They can extend the thermal range of your sleeping system by acting as a base-layer and they can help to keep your sleeping bag clean. They are quite packable and are a great way to extend the life of your bag.
Liners come in a range of different materials that greatly affect their performance. The lightest and most packable of which is silk. Silk liners are easily washed and feel nice to sleep in which make them ideal for those using them mainly for the protection of their sleeping bag. Cotton liners provide the same benefits but are usually heavier and bulkier. Despite this they are often bought as the cheaper option for use with school groups. The third material liners are often made from is synthetic fabrics. Synthetic fabrics offer a fair bit of insulation which is the main reason for their use. Finally there are fleece liners which are less common but offer the greatest increase in warmth. These liners are usually bulky and heavy but are mostly used by mountaineers looking to extend their sleeping range.
Pillows are a comfort item that can contribute greatly to a good night’s sleep. While they do technically contribute to the insulation of the head from the ground, this is not really their role as they are generally placed on top of a mat. There are three main types of constructions that balance comfort and packability. The most comfortable options tend to be the foam filled pillows as these provide a softer feel, closer to that of a regular pillow. These do not compress very well however. Air filled pillows can pack very small and be very light but they are firmer and not as comfortable as the foam filled pillows. The compromise between these is an air filled pillow with a foam layer on the upward side. One example of this is the Exped REM Pillow. These pillows optimise both comfort and packability and tend to be the best option for hikers.
Having the right sleeping equipment for the environment is crucial in having a comfortable night. This can be really important when you are spending back to back days in the wilderness and need to wake up and perform every day. The right balance of warmth, comfort, packability and weight is easiest achieved when you consider how your equipment works as a system, rather than individual items.
By Daniel Butler