Sleeping bag temperature ratings are intended to help you choose the right bag for your conditions, but they are not a guarantee that every person will feel comfortable at the listed temperature. Warmth can vary based on your metabolism, clothing, sleeping pad, shelter, weather conditions, and whether you naturally sleep warm or cold.
What do the temperature ratings mean?
Standardized sleeping bag testing may provide several different ratings:
Comfort Rating
The Comfort rating represents the lowest temperature at which a person who generally sleeps cold should be able to sleep comfortably in a relaxed position.
Cold sleepers should pay the closest attention to this number.
Lower Limit Rating
The Lower Limit represents the lowest temperature at which a person who generally sleeps warm may be able to sleep without becoming excessively cold.
This does not necessarily mean the sleeper will feel comfortable. They may need to curl up, wear additional layers, or otherwise supplement the warmth of the bag.
Extreme Rating
The Extreme rating is a survival rating—not a comfort or recommended-use rating. It indicates conditions where there may be a serious risk of hypothermia.
Customers should never plan a trip around the Extreme rating.
How are sleeping bags tested?
Sleeping bags are evaluated using a standardized laboratory test. A temperature-controlled manikin wearing base layers is placed inside the sleeping bag on a sleeping pad. The test measures how much energy is required to keep the manikin warm as the surrounding temperature decreases.
This testing provides a useful way to compare sleeping bags, but it cannot account for every person, environment, or sleeping setup.
Why doesn’t my sleeping bag list every rating?
When a specific temperature rating is not listed, it generally means one of two things:
- The sleeping bag has not been tested for that rating.
- The standardized test cannot produce a sufficiently accurate or reliable result for that temperature range.
For example, NEMO does not publish Comfort or Lower Limit ratings for the Sonic™ 0°F and Sonic™ -20°F sleeping bags. Standardized temperature testing becomes less accurate at approximately 0°F and below, so we do not publish ratings that could give customers a misleading expectation of performance.
The temperature shown in the product name may therefore be the intended or recommended-use rating rather than a complete set of laboratory-generated Comfort and Lower Limit results.
Which rating should I use?
NEMO uses different temperature ratings for men's and women's sleeping bags:
- Women's sleeping bags are named and marketed using the ISO Comfort rating, which is designed to represent the lowest temperature at which a person who generally sleeps cold should remain comfortable.
- Men's sleeping bags are named and marketed using the ISO Lower Limit rating, which represents the lowest temperature at which a person who generally sleeps warm may sleep without becoming excessively cold.
If you generally sleep cold, regardless of gender, we recommend choosing a sleeping bag with a lower temperature rating than your expected overnight lows or considering a women's model, which is rated using the Comfort standard.
If you generally sleep warm, a men's bag rated using the Lower Limit may be appropriate, but we still recommend leaving a safety margin between the forecasted nighttime temperature and your sleeping bag's rating.
For example, if temperatures are expected to reach 30°F (-1°C), choosing a bag rated comfortably below that temperature provides additional protection against changing weather, fatigue, damp conditions, and individual differences in metabolism.
Your sleeping pad matters
A sleeping bag insulates you from the air, while your sleeping pad insulates you from the ground. A warm sleeping bag cannot fully compensate for a sleeping pad that does not provide enough insulation for the conditions.
Temperature testing assumes the use of an insulated sleeping pad. Always make sure your pad’s R-value is appropriate for the expected nighttime temperatures.
Other factors that affect warmth
You may feel colder than the listed rating because of:
- An insufficiently insulated sleeping pad
- Damp clothing or sleeping bag insulation
- Wind or high humidity
- Hunger, dehydration, or exhaustion
- Wearing too many tight layers that restrict circulation
- A sleeping bag that is too large and contains excessive empty space
- Natural differences in metabolism and cold tolerance
Temperature ratings should be treated as a starting point—not an exact promise of comfort. Check the forecast, understand your sleeping habits, and leave an appropriate safety margin when selecting your sleeping bag.
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