The second thing you’ll notice when you’re searching for a sleeping bag is the temperature rating (the first thing might be the cool name, maybe a greek god or native spirit — like cars, don’t be swayed). Worst case: you can expect to survive at that temperature, but sleep will seem a far off thing like melting marshmallows. This is the survival rating. Better case: this is the so-called comfort rating and you’ll probably be able to sleep despite the cold feet. Clearly, there is a big difference between the two temperatures.
Best case: some companies give both numbers. Too often though, a number is thrown around without reference to where it stands on the survival/comfort spectrum and, never, for whom. I would expect (hope) the numbers are for the average male, maybe average female for the women’s bags; but who’s average? the outdoorsman? or, maybe pushing the survival end with a burly “hot” outdoorsman with ample “cold training” — I’ll explain “hot” and “cold trained” and it has nothing to do with melting “marshmallows” or potty training sled puppies.
Next thing to notice is the material of the sleeping bag, inside and out. I’m a diehard down lover (of the bird variety) because the insulation per weight is incredible and it compresses to itty bittiness. Fast and light gets tossed around a lot. This is as far as I’ll go down that slippery slope. The alternative to down is a variety of synthetic fills. These are almost always heavier and degrade faster. BUT, unlike down that squashes down like a poor kitten sodden in the gutter, synthetic fills retain some of their insulation when wet. I’ve tested this. Really, a wet down bag will suck the heat out of you, literally (to work on evaporating all that water), but a wet synthetic bag will cozy up warm and as soft as wet nylon can ever be.
On the outside, there’s a hierarchy of shell fabrics, just like in shell jackets, from simple nylon up to Gore produced (and competition) levels of waterproof-breathable fabrics. Since there’s a lot more fabric than a jacket, weight accumulates faster, so you probably won’t find fancy three (and four?) ply fabrics (more layers to control the one-way moisture flow: you want water to go against the thermal graduate from your hot body to the cold outside).
In the night, we sweat; under duress, we might sweat a lot. Remember those down bags? You need to get that moisture out before it dampens your insulation. In a tent, there might be condensation on the walls (hot, moist air meets cold outside): open the tent flaps high to let the hottest (moistest) air out. If it’s raining outside, pressure against the tent will undo whatever waterproofness it promised (nothing is truly waterproof — apply enough pressure/time and water will pass though). If it’s feeling crowded, or I’m on a slope, I’ll put my waterproof jacket over my feet (this probably doesn’t help, but it makes me feel better at night).
As a brief aside, you may hear suggestions of putting your wet clothes in the sleeping bag with you to dry off. The extra heat from your body (if you have any to offer) will evaporate that wetness and a good bag will pass that through to the tent. Do not attempt this if you’re tired and cold. It’ll just make you colder, wetter, and probably won’t work anyway. The most I’ll take into my bag at night are damp socks and liner mitts. For the rest, when you stop to make camp, layer up. This is good advice regardless. Stay warm and keep moving. In between layers — usually just after my thinnest insulation layer (a thin fleece) — plaster any damp clothing you aren’t already wearing, usually just the spare liner mitts for me.
To wear or not to wear clothing inside your sleeping bag
Far too often, I hear recommendations against wearing clothing in the sleeping bag. “They’re designed to be slept in naked.” In all the literature I searched, experiments involving people or dummies always involved some clothing. Don’t wear something that will block heat and moisture flow, but extra insulation is still extra insulation.
Men vs women: more than just size and shape
Not only is the average man larger and heavier than the average woman (meaning more heat production which goes with volume, but also faster heat loss which goes with area), but we physiologically manage our heat compromises differently. Women are far faster to sacrifice the extremities to keep the core warm (cold feet sound familiar?). Men, even accounting for higher metabolisms given their size, continue to metabolize higher relatively when they sleep. How often you expose yourself to cold, awake and sleeping, can increase your tolerance to sleeping cold and raise your sleeping metabolism (so does eating a chocolate bar in the middle of night, teeth be damned). This is cold training. Don’t ask about the nearly naked peddling rectal temperature experiments — the Norwegians are nuts.
Here’s a little math. The insulation, I, of a sleeping bag is measure of the steady state (no net heat flow) temperature gradient it can maintain over a certain volume divided by the heat source within the bag (you): I ∝ Δ T A / W.
Insulation increases with the loft of the bag (notice how a good winter bag looks like there’s already somebody inside of it when it’s laid out). Ideally, the (outer) area is minimized around the person while maintaining a continuous hefty loft of insulation around them: get the right size of bag.
A “hot” man metabolizes 50% more heat than a “cold” one and 33% more heat than a “hot” woman; a “hot” woman metabolizes 30% more heat than a “cold” woman and 10% more heat than a “cold” man. All these percentages translate proportionally to tolerable temperature ranges. An at-home sleeping skin temperature is around 30°C; a tolerably colder one is 25°C. A summer sleeping bag rated to freezing for a “hot” man will only give the equivalent insulation at 15°C for a “cold” woman (where “cold” could just mean small).
Men or women specific bags never fit me right: there’s always too much room around the core. To fill that space, I stuff every over down insulated item I carried (usually just a down jacket, but sometimes two of them). For truly excessive space and the remarkably strong, two sleeping bags, one inside the other, can work; note, there’s no point in two bags if the fit is tight enough to squeeze away the loft that either would deliver on its own.
Sleeping side by side shares heat, even if it’s not close enough to compress loft. If you squeeze tightly, the inside temperatures will try to equilibrate — so your tent partner can share some of his/her hotness.