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Gear Review: Big Agnes Sidewinder SL 20 Sleeping Bag

Gear Review: Big Agnes Sidewinder SL 20 Sleeping Bag

Posted by Dan Goodman on 9th Jun 2021

Your sleeping bag might be the most important part of your camping kit. If you forget your tent at home, you sleep in the open air. If your sleeping pad deflates at camp, you might be uncomfortable, but you’ll make it through the night.

But without your sleeping bag, you’re probably driving back home and calling it quits on your camping trip altogether.

And since it’s your most important piece of gear for sleeping under the stars, it’s also vital that your bag conforms to your personal sleep style, not the other way around.

Sidewinder SL 20 Sleeping Bag Label

The Big Agnes Sidewinder SL 20 Down Sleeping Bag is made specifically for side-sleepers, which, as it turns out, is a lot of us. With a centered zipper, extra insulation in high-pressure areas, and a technical fit designed to move with you if you switch sides mid-slumber, the Sidewinder SL is intended to give you the best night’s side-sleep you’ve ever had outdoors.

In theory, this lightweight sleeping bag is exactly what you’ve been waiting for all these years. But just how does it perform when you get it outside?

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Side-Sleeping Fit

Tossing out the idea of using a left or right-handed zipper, the Sidewinder sports a centered zipper, making it easy to get into and out of the bag regardless of which side you’re sleeping on. This feature also ensures that you won’t be laying on an uncomfortable zipper all night. Big Agnes also added space in the shoulders, hips, and foot of the bag to give you more room to maneuver and to accommodate legs stacked on top of each other. And these proportions are designed to move with you if you toss and turn at night, meaning you won’t just roll around inside of your bag or wake up with it twisted all around you.

Diagram highlighting areas of additional synthetic insulation in the Sidewinder Sl 20

Body-Mapped Insulation

Every side sleeper knows that while camping, you’ll inevitably wake up with numb hips or feet from the added pressure of sleeping on your side. Most lightweight down bags are filled entirely with down insulation, which compresses readily. While this is a great feature for packable sleeping bags, it doesn’t bode well for side-sleeping comfort. In the Sidewinder SL series, Big Agnes added a layer of less compressible, synthetic insulation along with the 650-fill down insulation at the hips and feet for additional padding. Since synthetic insulation isn’t as compressible as down, it cradles your hips and feet in a comfortable cushion, ensuring you won’t feel the hard ground underneath. This not only reduces unwanted pressure points but gives you additional warmth right where you want it.

Size and Temperature Options

We chose the hybrid down-and-synthetic Sidewinder SL 20 for a few car camping trips in the late spring and early summer. That particular configuration works well for us, but for those with different needs, the Sidewinder SL is available in men’s and women’s models, and both can be had with either a 20℉ or 35℉ temperature rating. If you don’t need the low weight and compressibility of the down hybrid design, there’s also the price point-friendly Sidewinder Camp series, made using 100% FireLine™ ECO Synthetic Insulation, and which gives you the same set of options.

Camper side-sleeping in a Women's Sidewinder SL Sleeping Bag

At the Campsite

Traditional mummy bags were designed with back sleepers in mind, and while they don’t stop you from rolling onto your side, they don’t make it easy either. Needless to say, we were excited to see what it was like to be able to sleep on our sides in a bag that’s made for the job.

Camper sleeping on their side in a Big Agnes tent

As promised, the Sidewinder SL 20 comes complete with an ambidextrous zipper down the center, generous legroom, and a unique fit compared to our other mummy bags. But one of the first things that grabbed our attention was the zipper. After years spent fighting zippers that get snagged on anything from thin layers to draft collars, this special zipper head was highly appreciated every time we used it.

Close up of the anti-snag zipper head

Once in the bag, the added insulation in the hip area was immediately noticeable. You can actually feel the difference in the baffles that contain the additional Fireline ECO Synthetic insulation. On the other hand, we didn’t notice the difference in the foot box, but it's reassuring to know you’re getting extra loft for your feet as well, even if just for extra warmth.

The centered zipper was also a huge plus and had us wondering why sleeping bags continue to use left and right-sided zippers. Better yet, this design also has an additional zipper head at the bottom to let you kick a leg out if things get too hot at night. We were a little disappointed that it’s only a ¾-length zipper instead of a full-length zipper, but it’s not enough to be a deal breaker.

Partially unzipped two-way zipper

Overnight, the bag performed just as you’d hope. It stuck with us as we rolled from side to side, eliminated the usual pressure points, and importantly, was incredibly easy to get into and out of, even when we were half asleep.

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What We Like

  • Body-Mapped Insulation: This is a huge bonus for any side sleeper. For those of us who like to stay on one side throughout the night, not waking up multiple times with a numb hip makes a noticeable difference. That plus the benefit of toasty warm feet makes this one of the most valuable additions to this sleeping system.
  • Patterned Fit: When we read that Big Agnes designed this specifically to be able to stay with you if you toss and turn at night, we weren’t sure how different that would be from a standard mummy sleeping bag. But after putting it to the test, it makes a difference. Our standard mummy bags were a bit too loose and, more often than not, we ended up facing the inside of our bags at some point during the night.

Anti-Snag zipper

  • Anti-Snag Zipper: Other sleeping bags might claim to have an anti-snag zipper, but this is the best we’ve ever seen. You have to try to get the draft collar stuck to get it to snag at all. This makes it a whole lot easier to answer the call of nature in the middle of the night.

What Could Be Improved

  • Foot Box: If you have big feet, you’re probably going to feel a little restricted by the foot box. It won’t make it impossible to sleep, but you’ll have to get your feet in a specific configuration and stay that way. And while Big Agnes did make the foot box slightly larger in this bag than in their mummy bags, we think a little more room would be an improvement.
  • Shoulder Insulation: We love the idea of the body-mapped insulation so much that we’d like to see it expanded to include the shoulder area as well. Hips and shoulders take the brunt of the pressure while side-sleeping, and we still found ourselves waking up with a numb arm every once in a while. This would add a bit more weight and bulk to the Sidewinder, but we think the benefits would outweigh the drawbacks.

Deflated camp pillow being put into the pillow barn

  • Pillow Barn: We’re generally big fans of Pillow Barns, but this one isn’t executed as well as the versions found on some of the other Big Agnes sleeping bags. You can only access it through two small pockets inside of the hood, which are too small to fit an inflated pillow through. This means you have to put your camp pillow into the Pillow Barn and then inflate it, which doesn’t work for every camp pillow. It’s also hard to find these pockets in the middle of the night to shift your pillow from side to side if you decide to roll over.


This Sidewinder SL 20 has a lot going for it, and not just because it’s unique. For a relatively lightweight and packable sleeping bag, Big Agnes managed to keep the price reasonable. They packed it full of features that side sleepers will love, and it’s just as comfortable if you choose to sleep on your back.

With that said, who should buy this sleeping bag? Any new piece of gear has an environmental impact, not just a financial one, so we don’t think it’s worth ditching a perfectly good mummy bag for this new design. But if you’re due for a new sleeping bag and you’re a side sleeper who needs a solid, all-around bag, we’d strongly recommend putting the Sidewinder at the top of your list.

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About Campman

Campman is an outdoor retail store located in Salt Lake City, Utah, that specializes in gear and equipment for camping, skiing, backpacking, climbing, and everything in between. We understand that the difference between a bad memory and the adventure of a lifetime can come down to the gear you take with you when you head outdoors, and we want our customers to have the best experiences possible. When it comes to getting outside, we get it, and we’ll get you the gear you need to get there.

Questions? Feel free to contact us by phone at (801) 999-8117 or by email at

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