Start with the temperature rating, not the price tag

Every sleeping bag sold in the UK carries a temperature rating based on the EN 13537 or ISO 23537 standard. You will usually see four figures: upper limit, comfort, lower limit and extreme. The one that matters most for most people is the comfort rating, which is the temperature at which an average woman can sleep comfortably. The lower limit is the temperature at which an average man can sleep for eight hours without waking. The extreme rating is a survival figure only — ignore it for planning. If you sleep cold, or you are shopping for a woman, work from the comfort rating. If you sleep warm, the lower limit may be more realistic. As a rule of thumb for UK hillwalking, add five degrees of margin to whatever the label claims. A bag rated to zero will feel chilly at five on a damp Cumbrian night.

Down or synthetic? Match the fill to the forecast

Down fill gives the best warmth for its weight and packs down small. It also lasts for many seasons if cared for. But it loses loft when wet, and once a down bag is soaked it is miserable. Synthetic fills — usually polyester — keep some warmth even when damp and dry faster. They are heavier and bulkier for the same warmth, and they degrade more quickly with repeated compression. For British conditions, a good compromise is a hydrophobic down bag, which has a water-repellent treatment on the feathers. If you mostly camp in wet western Scotland or the Lake District in autumn, synthetic is the forgiving choice. For crisp, cold winter bothies and snow holes, down wins on warmth-to-weight every time.

The mat beneath you matters more than you think

You lose far more heat to the ground than to the air. A sleeping bag's fill is compressed under your body, so it provides almost no insulation there. That job belongs to your sleeping mat. Look for an R-value of at least 3 for three-season UK camping, and 5 or more for winter. A closed-cell foam mat is cheap, light and indestructible, but bulky. An inflatable mat with synthetic insulation is comfortable and warm, but can puncture. Many hillwalkers carry a short foam mat under the torso plus an inflatable mat for comfort. Whatever you choose, never sleep directly on the ground sheet. A reflective emergency blanket under the mat adds a surprising amount of warmth for very little weight.

Shape, hood and draft collar

Mummy bags are the most efficient shape because there is less air to heat. They can feel restrictive if you toss and turn. Rectangular bags are roomier but heavier and less warm for the same fill. A good hood traps heat around your head — a huge amount of body heat escapes from an uncovered head. Look for a draft collar that seals around your shoulders and a draft tube behind the zip. A two-way zip lets you vent on milder nights. Left or right zip matters if you want to zip two bags together with a partner; check the side before you buy. If you sleep on your side, choose a bag with a bit more shoulder width or a quilt-style opening.

Think in seasons and layers

Most UK hillwalking from late spring to early autumn suits a three-season bag rated around 0°C to -5°C comfort. For winter, you want a bag rated to -10°C or lower, plus a warmer mat and often a liner. In very cold conditions, you can layer a lightweight synthetic bag inside a down bag, or use a top quilt over the top. A silk or fleece liner adds a few degrees and keeps the bag clean. Here are a few habits that make a real difference:

• Sleep dry: never wear the clothes you walked in; damp fabric pulls heat away.
• Cover your head: a warm hat traps heat that would otherwise escape.
• Eat before bed: a handful of nuts or a flapjack gives your body fuel to generate warmth.
• Pre-warm the bag: do a few star jumps or fill a bottle with hot water and tuck it in.

Care, storage and keeping it dry

Never store a sleeping bag compressed. Down and synthetic fills need to loft. Keep it in a large cotton or mesh storage sack hanging in a dry cupboard. When you wash it, use a gentle non-detergent soap and a front-loading machine on a cool cycle, then tumble dry on low with a couple of clean tennis balls to break up clumps. A damp bag loses most of its warmth, so pack it in a waterproof stuff sack or line your rucksack with a dry bag. Air it every morning before you pack, even if only for ten minutes. If it does get wet, dry it thoroughly before storing. With care, a good bag will last a decade or more.