How Do Sleeping Bags Work?

The main purpose of camping is wanting to be one with mother nature. Exploring, hiking, and taking in all the greenery can tire one out really quickly, and that makes rest an important part of every trip, not just camping. If you don’t get a good night’s sleep, you won’t be rejuvenated enough to take on another day of roughing it out. A sleeping bag is the most feasible solution to provide just that – the gateway to sleeping peacefully outdoors.

That being said, not all sleeping bags will do what is promised. Waking up in the middle of the night all curled up inside your tent because the cold is unbearable and your sleeping bag isn’t doing enough isn’t an ideal situation. That’s why choosing a sleeping bag based on its temperature rating is important. If you have a rough assumption of what the weather will be like where you’re headed to, you can pick a sleeping bag that has a temperature rating capable of keeping you warm. Before getting into further details about choosing better sleeping bags, let’s know a little bit more about how these bags actually work.

Sleeping bags are an essential part of camping, but do you ever wonder how they work?

The Science Behind Sleeping bags

One of the ways we can protect our bodies from losing heat in cooler temperatures is by wrapping ourselves with much-required insulation. This constricts a layer of air around the body and stops it from circulating. The air is then warmed by the heat produced through our bodies. The bag creates a sort of barrier between the surrounding air or colder ground and the “dead air.” Heat is more efficiently retained in smaller areas, which is why camping sleeping bags are much compact – well, that and the fact that it needs to be carried quite frequently. As a result, the body doesn’t lose as much heat.

Like sleeping bags, winter coats also function on the laws of insulation. The two main insulation materials in sleeping bags are synthetic insulation and feather down or simply down.

What are Sleeping Bags Made of?

Sleeping bag Fill Materials


A down sleeping bag offers maximum insulation. It’s extremely compact and folds very well; but when unrolled, it significantly fluffs up. High-quality downs typically have a fill rating of 500-800 inches per ounce.
These sleeping bags are incredibly soft and get a satisfactory score on the warmth-to-weight scale. The only downside to the downs is that they’re not the best in humid or wet conditions, as downs don’t function when wet.


Synthetic-filled versions were created to incorporate the pros of down with improved insulation features if the sleeping bag gets wet. Nevertheless, these don’t compress as well as their counterparts, but they are machine washable and dryable. Synthetic-fill sleeping bags are the best choice for camping and hiking in the warm summer.

Lining Fabric

Polyester, Nylon, and Taffeta
These materials are the most popular lining fabrics for sleeping bags, thanks to their soft, smooth texture. They’re super breathable and comfy, and they heat up less compared to others on the list when you’ve been on the same spot for a long time. Cheaper camping sleeping bags come with polyester or nylon linings. On the other hand, taffeta is the most premium grade out of the three.
Cotton or Flannel
Most popularly used in rectangular sleeping bags, the natural materials of cotton make it lightweight, breathable, and, at the same time, durable. They’re easy to clean, and patch jobs are also pretty simple. Flannel and cotton trap moisture, so they’re ideal for moderate to dry climates.
If you have some extra bucks to splurge on some amazing quality sleeping bags, you’ll find that those typically feature silk linings. Silk fabric is supple, soft, and ultra-breathable. You won’t feel excessive discomfort as it doesn’t overheat from capturing the warmth of the body.
On the minus side, it’s vulnerable to tears and much harder to repair. Now, since most people look for a rugged sleeping bag that will sit nicely even on some bumpy surfaces without budging, the silk’s delicateness can be an issue. It’s great to sleep on otherwise.
Fleece fabric and more brushed lining ones are better when the temperature starts to decline. They’re soft to the touch. However, since this fabric does trap a generous amount of heat, it could get a bit uncomfortable when you’ve been lying in the same spot/position for a while. You’ll mostly see these in sleeping bags that offer more room to shift around in – rectangular sleeping bags, for example.

What are Sleeping Bag Temperature Ratings?

We talked about temperature ratings in the first section, and here we go into the nitty gritty. In warm, almost tropical climates, or at lower elevations, opting for a summer bag is the most practical solution. It saves weight, money, and valuable space inside the tent. There are some sleeping bags that are designed for all seasons, known as 3-season sleeping bags.
The core issue is the temperature dip at night in the high mountainous regions. For camping in the chilly seasons, a winter bag is absolutely needed. The perfect temperature rating depends heavily on the weather conditions, but some cold-weather expeditions can run down as far as -40°F.
Summer/low-elevation sleeping bags: 32°F and over.
Cold-weather/winter sleeping bags: 20°F and under.
3-season sleeping bags: 20° – 32°F.

How Sleeping Bag Designs Affect Warmth

Apart from the temperature rating, factors like features and shape play a key role in determining the warmth giving capacity of the sleeping bag.
A lot of people choose roomier bags because they prefer having the freedom to toss and turn as much as they want. They should remember that the more dead space there is, the more energy (body heat) is required to heat up the inside of the bag. Always pick one that hugs your body and is comfortable without having too much free space. Side sleepers will still need space inside their bags, but this is a personal preference.
“Roomier” bags do run the risk of being somewhat colder than their ratings, but it’s only due to the empty space. In some cases, there’s a warmth and comfort tradeoff existent between the bags, but we believe that there are models that satisfy both needs.

Additionally, styles influence the warmth of camping sleeping bags. For instance, sleeping quilts are a top choice for some thru-hikers. The sleeping quilt cuts off near the shoulder or neck region instead of going around the entire head like traditional sleeping bags. While this does save space and reduce weight, it comes at the cost of comfort.

Newer backpackers will want to choose something that’s much easier to use, as thru-hikers might even pack an extra light down jacket just for sleeping. It’s meant more for the ones who’ve grown a thick skin in the wild. A mummy-style bag with a draft collar and tube, adequate hood, and insulated bottom will do nicely in this case.

Last but not the least, the older the sleeping bag, the less effective it will be. Some models are definitely more long-lasting than others, but the power does wear out over time.


The bane of every backpacker’s camping journey is the complex relationship between weight and money of sleeping bags. Lighter ones are cheaper, but heavier ones are warmer. There’s always the question about how to choose one, but we suggest not cutting it too close as it concerns safety and sleep.

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Daniel has experience camping across California in a variety of environments and parks. His interests in technology lead him to focus on writing about camping innovations.

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