The best person to answer this question is the person who will be using the sleeping bag, after considering what it will mainly be used for. A brilliant bag for extreme conditions has very different attributes to those of a lightweight travel bag. Warmth, weight, size and features are all very important, with the paramount virtue dependent on the end use.

Sleeping bags can be broken down into four main types: alpine, bushwalking, travel and general use bags, although categories do overlap.

Alpine bags are designed for extreme conditions above the snowline. They need to be both warm and light: a mummy shape, lightweight fabric and high quality down are used to give a high warmth-to-weight ratio, with water-resistant shell fabrics also common.

For bushwalking bags, the focus is on both performance and comfort. These bags are still light but have a few extra features and allowances for comfort, and may not use such high-specification materials. Bushwalking bags come in both mummy and tapered rectangular shapes.

There is less emphasis on technical features and warmth with travel bags, as they are generally used indoors. Packed size is still important, as is versatility and comfort, so most bags are tapered rectangular in shape.

General use bags are a little heavier, great for general camping, sleepovers, school camps and extra bedding at home, and are designed to be comfortable, functional and durable. Size, weight and technical features are not a priority, so most bags are rectangular or tapered rectangular in shape and robust in construction.

Sleeping bag shapes

Sleeping bags shapes are designed to strike a balance between comfort and performance, taking into account the priorities of the users. The bigger a bag is internally, the more room there is to move but the more air there is to heat. Any increase of space in a bag allows more wriggle room, but also reduces the bag’s thermal efficiency.

Sleeping bags come in three shapes: mummy, tapered rectangular and rectangular. The most thermally-efficient sleeping bag shape is the mummy. This design is wide at the shoulders with a pronounced taper to a narrow foot, reducing the amount of space – and air to be heated – within the bag. These bags offer maximum warmth for minimum weight; perfect for the adventurer looking for a compact, technical bag for their outdoor pursuits.

The narrower fit reduces the ‘wriggle room’ within the bag, which some people may find restrictive. Try one out before you buy, particularly if you’re claustrophobic or like to thrash around in your sleep! Many mummy-shaped bags have half- or three-quarter zips, designed to finish at the knee or calf, reducing the bag’s weight and increasing its warmth. However, a full-length zip gives more versatility as you can open the bag up and vent your feet in warmer conditions, and it also allows easy entry and exit.

Tapered rectangular bags are what they say…They’re less fitted than a mummy bag, but taper to the foot, reducing dead space and the air to be heated within the bag. This style of bag is a good all-round choice as they are comfortably roomy – and can be fully unzipped to make a quilt for your bed – while still being efficient enough to pack down, not tip the scales and keep you warm.

Rectangular bags are roomy and comfortable, giving plenty of space for moving around. Designed for uses where warmth, packed size and weight are not critical, they are generally larger, heavier and less technical bags.

Sleeping bag fill

A sleeping bag’s fill insulates you from the cold environment: it traps body heat and prevents the exchange of warm and cold air. It’s simple really, although it’s easy to lose sight of this when boggled by technical details about loft, feather/down composition, fill weight and synthetic suspension systems. At its most basic, it comes down to two choices: down (natural) or synthetic fill.

Good quality down is still the most efficient sleeping bag insulation available, with the best warmth-to-weight ratio and amazing compressibility. With proper care, down lasts a long time, maintaining its performance better than synthetic fibre. On the flip side, down is expensive and not exactly easy-care. When wet it loses most of its insulation value, and it can take a long time to dry.

Synthetic fill is also an excellent option. It is easier to clean, maintains greater insulation when wet and is less expensive than down. However, it is also bulkier, heavier and may have a shorter user life.

It is worth noting that sleeping bag fill works most efficiently when clean.