For a full guide on how to choose the best sleeping bag, read our guide.
You can buy a sleeping bag which is for One Season (summer), Two Seasons (summer and autumn), Three (Spring, Summer, Autumn) or Four Seasons. Four+ Seasons is also available, but this is usually for non-UK conditions.
Temperature ratings will be expressed in comfort (lower and upper limits) and extreme ratings.
The comfort rating refers to the temperature you can use the bag in and still will feel warm and comfortable when in a rolled-up position. When the bag is used in any temperatures below this, you are likely to feel the cold. Most sleeping bags will just use the ‘lower’ limit here, but those which keep you very warm through 4 seasons will often have an ‘upper’ one too.
Extreme ratings are also used, which is the ‘survival’ temperature. It is essentially the temperature at which the sleeping bag will keep you alive without frostbite. However, you should not use the sleeping bag at this temperature if it can be avoided.
Men usually sleep at cooler temperatures to women as they are less likely to feel the cold, so there is usually a bit of leeway here, but they are only a guide.
You have two options here – down, or synthetic.
The former is usually duck down. The loft creates little air pockets which makes it more effective at trapping heat. Because they don’t need to be filled as much, they are lighter and pack up smaller. The warmth-to-weight ratio of a down sleeping bag cannot be beaten by a synthetic bag. But a big disadvantage is that it absorbs water when damp, ruining the heat properties and taking ages to dry.
Synthetic is the most common, and will also be cheaper and require the least attention in terms of care. They also perform better in wet conditions, retaining some of their heat. But they will usually be thicker and heavier, as they need more filling in order to work at the same temperature levels.