A comfort or season rating on a sleeping bag is there as a guide only and should be used in conjunction with other key factors to ensure you balance price, weight and bulk with a good night's sleep. Unless you can find a sleeping bag that has an inbuilt heating system a sleeping bag by itself cannot heat you up. The only heat source within a sleeping bag is you. A sleeping bag only provides insulation to ensure any heat you generate only heats the air around you.
So let's break it down, first we need to look at ourselves and ask some simple questions.
If you are the sort of person that needs an extra blanket when your partner is sweating under a summer duvet you are going to need more insulation from your sleeping bag and will probably need a bag with a lower comfort rating.
If you crawl into a sleeping bag tired and hungry after a hard physical day you body isn't going to have the energy it would normally have to continue keeping you warm through the night. Again if you're not used to physical exertion, especially at altitude you might need to look at sleeping bags with lower comfort ratings.
A sleeping bag retains your bodies heat by trapping warm air around you. If you wriggle and move about a lot during sleep your sleeping bag may act like a set of giant bellows, pushing warm air out and sucking cooler air in. Sleeping bag baffles helpreduce this but again you may need a sleeping bag with lower comfort rating. A few technical sleeping bags have less insulation on the base than the top. The theory being that insulation is crushed when you lie on it so why bother with putting as much there. If you move around during sleep the panels with less insulation could end up being on top of you and the fuller top sections crushed underneath you.
If you like the sun and can be found huddled up by the fire when the mercury starts to drop anywhere near zero, chances are you're not a fan of the cold. If you're heading off somewhere colder than you are used to your body will probably react more severley to this and again you may need to look at sleeping bags with lower comfort ratings. However if you can expose your body to lower temperatures, especially your extremities, and acclimatise them the more comfortable you'll feel when you eventually get there. Before our extreme bike rider Paul Errington set off on the Arrowhead Race in Alaska he sat in front of the telly with his feet and hands in buckets of iced water.
So now we know a little more about how we sleep we now need to look at the climatic conditions we'll be sleeping in.
In the UK our weather tends to be humid and this can make temperatures around zero feel much colder. In the Alps and other large mountain ranges the air tends be very dry and for some sleeping at very low temperatures can feel similar to temperatures in the UK of 10-15ºC warmer.
Sleeping on the top bunk probably won't have much effect butif you're sleeping at altitude even one night your body will be using a lot more energy to keep itself going. To compensate you are going to have to make sure you take on extra fuel (food and water) to keep it running warm.
Conduction through the ground is the quickest way to lose heat whilst sleeping. If you are going to be sleeping on frozen ground or snow it is probably worth spending some money on your sleeping mat as well as your sleeping bag.
Now let's ask some questions about how you are going to use the sleeping bag.
Most people think their sleeping bag needs to pack up as small as they can make it. For some this is an important factor but for most we do it because that's the bag it came with. Simply put though the more you compress a sleeping bag the longer it will take for the insulation to recover to its best. A great deal of lightweight backpackers are starting to opt for larger, lighter rucksacks and rather than filling them with more kit they simply leave key insulation items like sleeping bags and down jackets as uncompressed as they can.
A wet sleeping bag will always feel cold. A wet "down" sleeping bag will have almost no insulation to it at all. This scares a lot of people away from down sleeping bags but in reality most people are able to keep a sleeping bag dry enough to retain the majority of its insulation. If you have tried to clean a sleeping in the bath you will know it takes a surprisingly long time to completely wet one out. What a lot of people forget or do not realise is that a sleeping bag retains a lot of your bodies perspiration, especially if used in a bivi bag. In warmer weathers this isn't a problem because the sleeping bag stays warm and this perspiration simply evaporates. However in sub zero conditions as soon as you crawl out of the sleeping bag there is no longer a heat source to evaporate that sweat enabling it to freeze around the insulation. One or two nights of this won't have a dramatic effect, but continued use without intervention will have a negative effect on the sleeping bags performance.
Asking yourself all of the these questions will help to ensure you are prepared for the conditions you may face and hopefully you will have chosen a sleeping bag adequate for those conditions. If you're pushing your sleeping bag towards its limit or you push your body harder than expected you'll hopefully understand why you may feel colder than expected. Last of all we need to look at the engine heating the bag, you! Unlike a car you can run on all kinds of fuels and you need to think carefully about what fuels you run best on. A chocolate bar will give us a quick hit of energy but it won't last through the night. In colder conditions, especially when you're tired or at altitude your body won't feel like eating or drinking so it is extremely important you monitor this and fuel yourself adequately. Shivering is your bodies involuntary response to cold and by firing your muscles quickly it helps generate heat. Falling asleep while shivering is extremely hard so if you feel cold before going to bed do some simple exercises before climbing into your sleeping bag. We're not talking a full workout here but a quick jog on the spot, some frantic arm flapping will help you physiologically when going to sleep.
Remember it doesn't matter how thick and fluffy your sleeping bag is... YOU WARM THE BAG!
Similarly tagged stickies: Cleaning Down Sleeping Bags, Left or right zip, SLEEP, Sleeping Bag Sizing, Which dry bag for my sleeping bag?, Which sleeping bag is suitable for my Kilimanjaro trip?, Why do I have 3 stuff bags?, Zipping together long and short sleeping bags,
Living with your Alpkit goodies and looking after them.
Some frequently asked questions and help topics to keep your Alpkit goodies in tip top order.