Sleeping Bags

If you're going camping, one of these is absolutely essential. You're not going to enjoy your excursion into the wilds unless you get a decent night's sleep, and that comes down to ensuring you're comfortably warm. Sleeping bags work by trapping dead air in the filling to insulate you. Most bags fall into one of two clear categories:

Down
The fluffy stuff which comes off the breasts of waterfowl is still hard to beat in terms of insulation for any given weight, although it does suffer from a drastic loss of performance if you let it get damp, let alone wet (these days it's possible to improve things by using proofing agents designed specifically for down made by companies such as Nikwax and Grangers). Still provides the best recovery from long-term compression, which is why the Royal Air Force vacuum-pack down bags into their fighter pilot seatpack survival kits.

Synthetic
To all intents and purposes, polyester fibres absorb no water, which has to be a big plus for a sleeping bag in a damp environment. Even when the bag is wet, it loses very little of its insulation value. Cheaper sleeping bag fillings - usually the unbranded polyester fibre waddings - have a limited lifetime, further reduced by frequent washing. Polyester fibre waddings are generally bulkier and heavier than the equivalent insulation value in down, and their lifespan is much shorter, particularly if the bag is stored in its stuff sack.
Having said that, many of the top quality polyester fillings come pretty close in softness, compression and recovery to the performance of down.

Seaon Ratings
Sleeping bags are often quoted as conforming to a particular season rating - 1, 2, 3, 4, and, oddly perhaps given the number of seasons in a year, 5, sometimes referred to as expedition. What it's trying to do is give you an indication of the temperature range the bag is good for at valley level. So if you planned to do a high-level camp in Scotland in the spring, what you'd more likely need would be a 4 season bag rather than a 3 season.
     1 Summer only, 10ºC
     2 Late spring, summer, early autumn, 0ºC
     3 Spring, summer, autumn, -5ºC
     4 Spring, summer, autumn, winter, -15ºC
     5 Expedition, below -15ºC
     Most manufacturers will quote the suitability of their bags in terms of minimum temperatures, some even giving the figures for what's comfortable, and what's tolerable - a distinction worth bearing in mind. And although they are good indicators, they need to be balanced against your own metabolism. What's comfortable for one person may only be bearable for someone else. Neither do such figures take into account the differences in comfort levels for any given temperature at different levels of humidity. You'll probably already know whether you're the kind of person who feels the cold easily, so take that into account when you make your choice.

Alternatively, stick with a lightweight bag providing minimum insulation, and simply uprate your bag by wearing extra clothes. Whatever you end up with, the golden rule is make sure you go to bed warm. Have a hot drink just before you turn in, or go for a quick jog around the tent to get the circulation moving.

Packability
To an extent, this comes down to the kind of insulation in your bag, but the amount of space which your sleeping bag commandeers in your rucksack can have a significant bearing on your total load. One useful advantage of having two ultra-lightweight bags is that, apart from simply using one when the weather is kind enough, if you use them doubled up, you can still pack them separately.
Some amazingly lightweight bags aren't so much lightweight because they've been made using some new space-age insulation - they've simply been cut small to use less materials. You don't have to be 6ft 5 and weigh 16 stones for this to be a problem, either, so pay attention to dimensions, and always take the opportunity to try the bag for size in the shop.

Features To Look For.........

Shell Materials
Lightweight densely woven fabrics are the norm, as down and even synthetic insulation fibres can percolate through even quite close-weave fabrics. A degree of water repellency is an advantage (stops any condensation drips from the tent getting too far). Linings vary depending on the weight and intended use of the sleeping bag. Cotton feels comfortable next to the skin, but for the same reasons that cotton underwear has long been abandoned in "performance" situations, it's now very much relegated to budget bags. Polycotton is a reasonable compromise, whilst lightweight bags will often use a variation on the shell material - Pertex is probably one of the most widely used.

Zip
The majority of sleeping bags come with full-length zips these days, and the better ones will have two-way pullers to enable you to ventilate the bag at the foot without completely undoing it. Bags like this will usually be available in left and right-hand zips so two bags can be zipped together (why would you want to do that?). But there are still some bags which come with short zips, and some without zips at all - a bit of a struggle to get in, but lighter and more thermally efficient than their zipped counterparts.
Where bags are zipped, they tend to come with a baffle behind the zip to prevent cold spots. The effectiveness of these baffles varies enormously, as does the way they avoid getting snagged in the zip itself. The best baffles incorporate some kind of anti-snag stiffening.

Neck Baffle
A useful feature if you're intending to do much cold-weather camping, as it reduces any bellows effect inside the bag, although it can feel uncomfortably restrictive to some. Not really necessary for 3 season camping.

Hood
Your head radiates heat more quickly than any other part of your body, so it makes sense to insulate it. At its most basic level, a hood is simply a flat extension of the underneath of your sleeping bag. The trouble is, it tends to bunch up horribly when you pull in the drawcord. Shaped or sculpted hoods do the job much more efficiently.

Foot Construction
The standard oblong construction of the cheapest sleeping bag does little to recognise the fact that human feet stick out rather than lie flat like Charlie Chaplin's. The majority of sleeping bags use a boxed foot construction, but there are variations on the theme which do the same job.

Construction
Quality construction counts for a lot. A synthetic bag usually has the insulation quilted to the shell fabric, and if it's a double layer, the top layer is stitched to the shell, the lower to the lining, with the stitch lines offset to prevent cold spots. Some of the more innovative manufacturers have come up with techniques which reduce the amount of such stitching, thereby improving the bag's thermal efficiency.
Because down is a loose filling, it's blown into pockets constructed between the inner and outer shells, the better ones using vertical or slanting baffles to keep the down in place whilst maintaining the separation which prevents cold spots.