A sleeping bag rated to 0°C will not keep most people comfortable at 0°C. That is not a lie on the label — it is a misreading of which of the three published numbers you are supposed to use.
The three numbers
Most bags sold in Europe are tested to EN 13537 or its successor ISO 23537. The standard produces several figures, and three appear on labels:
Comfort. The temperature at which a "standard woman" can sleep comfortably in a relaxed posture. This is the number most people should buy by.
Limit. The temperature at which a "standard man" can sleep for eight hours curled up, without waking. Colder than comfort, and the figure most often printed largest on the packaging.
Extreme. A survival figure. The temperature at which a standard woman can survive six hours without hypothermia, with strong shivering. It is not a sleeping temperature and should never be used to choose a bag.
The gap between comfort and extreme is often twenty degrees. A bag advertised as "-10°C" is frequently quoting the extreme figure, with a comfort rating near +2°C.
The standard sleeper is probably not you
The test uses a heated mannequin, and the standard bodies are defined as a 25-year-old man of 73kg and 1.73m, and a 25-year-old woman of 60kg and 1.60m.
Real differences that shift the useful temperature by several degrees:
- Women generally sleep colder than men, which is why the standard produces separate figures.
- Being tired, dehydrated or underfed lowers your heat production substantially.
- A larger person in a bag with lots of spare space has more air to heat.
- Sleeping without eating in the evening is the most common reason for a cold night in an adequate bag.
Treat the comfort rating as a starting point and add a margin for yourself.
The mat matters as much as the bag
Insulation underneath you is compressed by your body weight and stops working. Most of the heat lost on a cold night goes into the ground, not the air.
Sleeping mats are rated with an R-value, and it is additive — two mats stacked give you the sum.
- R 1–2 — summer only.
- R 3–4 — three seasons.
- R 5+ — winter, snow.
A -5°C bag on an R 1.5 mat will be a cold night. The same bag on an R 5 mat is a different experience entirely. If a bag feels colder than its rating, the mat is the first thing to check.
Down and synthetic
Down gives the most warmth for its weight and packs smallest. It is measured in fill power — 550, 700, 850 — which describes loft, not quantity. A bag with 850 fill power down needs less of it for the same warmth, so it is lighter and more expensive.
Down fails when wet. Treated (hydrophobic) down resists this for longer but does not solve it.
Synthetic is heavier and bulkier for the same warmth, cheaper, and keeps insulating when damp. It also loses loft with repeated compression faster than down does.
The honest rule: down for dry cold and weight-critical trips, synthetic for wet conditions, budget, and anything that will be stored compressed or washed often.
Shape
Mummy bags are the warmest for their weight because there is less air to heat, and the most restrictive.
Rectangular bags are roomiest and coldest, and often zip together in pairs.
Semi-rectangular splits the difference and suits most people who are not counting grams.
A bag that is too big is measurably colder. Length options exist for a reason.
Practical things that add several degrees
- Wear a hat. A surprising amount of heat leaves an uncovered head.
- Use the hood and the neck baffle. Most bags have both and most people do not close them.
- Eat something before sleeping. Digestion produces heat for hours.
- Change into dry clothes. Damp base layers will cost you the night.
- Vent before you sweat. Sweat in the insulation is the slow way to get cold.
- Store the bag uncompressed. Loft is warmth, and months in a stuff sack destroys it permanently.
The short version
Buy by the comfort rating, not the headline. Add margin for yourself. Then spend attention on the mat's R-value, because on a cold night the ground takes more from you than the air does.
More kit notes are filed under Outdoors & Travel.