The Therm-a-Rest Hyperion 20F sleeping bag is one of the lightest in its class. Weighing a mere 20 ounces, this slim-cut mummy bag has a half-length zipper, and a hood, and is stuffed with lofty 900-fill Nikwax Hydrophobic down. It’s pretty expensive, not very spacious, and only works for back sleepers, though.
Therm-a-Rest Hyperion 20F
Price: $469.95 to $579.95
✅ Less expensive than some ultralight bags
✅ Very packable
✅ Has a hood
❌ Only good for back sleepers
❌ Zipper snags easily
- Weight: 1 pound 4 ounces
- Packed size: 6 diameter x 8 inches
- Temperature Rating: 20* Fahrenheit
- Fill: 900-fill Goose Nikwax Hydrophobic Down RDS
- Fill power: 900-fill
- Fill weight: 12.5 ounces
- Length: 80 inches
- Shoulder Girth: 57 inches
- Hip Girth: 49.5 inches
The Therm-a-Rest Hyperion 20F Sleeping Bag is one of the lightest sleeping bags available. It’s a slim-cut mummy bag and is rated to 20 degrees Fahrenheit. This bag prioritizes weight savings over all else. It has a half-length zipper on the left side of the bag, a hood with a draft collar, and little else in terms of extra features.
It’s one of the narrower sleeping bags available, which is one of the ways this bag is made as light as it is. It also has most of its down on the top of the bag, which means if you sleep on your side, you’ll expose the less insulated side of the bag.
For this reason, this sleeping bag is really for back sleepers only who don't move around a lot. But if you toss and turn throughout the night, sleep on your side, stomach, or a combination of both, you’re better off getting a bag with more room to move around in.
Though this bag is rated to 20 degrees Fahrenheit, it is comfort rated to 32 degrees. This means when the temperatures drop below freezing, you’ll need some extra layers or a sleeping bag liner with this sleeping bag.
This is an excellent choice for summer ultralight backpacking trips and thru-hikes, as well as with milder shoulder season use. To see reviews on other ultralight sleeping bags, see our post on the best ultralight sleeping bags.
Similar Products: Zpacks 20F Sleeping Bag, Sea to Summit Spark Ultralight, Mountain Hardwear Phantom 15, Western Mountaineering Ultralite.
Performance Test Results
What We Tested:
How We Tested:
I have thru-hiked the Pacific Crest Trail, Continental Divide Trail, the Colorado Trail three times, and the Grand Enchantment Trail, along with countless shorter backpacking trips and thru-hikes. I tested the Therm-a-Rest Hyperion on a series of hikes and backpacking trips in Colorado during the fall and early winter seasons. The temperatures were cold at night, getting into the 20s on multiple occasions.
The Therm-a-Rest Hyperion weighs a mere 1 pound 4.7 ounces, according to my home scale. The packed weight of this sleeping bag is 1 pound 6.5 ounces. That weight includes the compression sack (1.6 ounces) and the Synergy Connector straps (0.2 ounces).
Any sleeping bag rated to 20 degrees Fahrenheit that weighs less than approximately 1.5 pounds is considered “ultralight.” The Hyperion is definitely an ultralight sleeping bag. It makes several sacrifices and uses the lightest materials available to achieve this low weight.
The warmth-to-weight ratio of this sleeping bag is very good, as you’d expect from a down sleeping bag. Down has the best warmth-to-weight ratio of any insulating material. In the case of the Hyperion, it weighs 20.7 ounces and is rated to 20 degrees Fahrenheit. So, the warmth-to-weight ratio of this sleeping bag is approximately 1:1. You’re not going to get much better than that.
It has a half-length zipper to save weight. The zipper runs along the left side of the bag and stops near your waist. I find it’s just long enough to crawl into the bag, but getting into this bag isn’t as easy as those with a longer zipper.
This uses 900-fill Nikwax treated down, the loftiest down available. 900-fill down is the warmest and lightest down available. Most of the down is in the baffles on the top of the bag. 70% of the down is on the top, and only 30% is underneath where your body will compress it. This is similar to a backpacking quilt but still allows you to zip the bag shut.
It uses ultralight 10D Nylon RipStop fabric for the shell and interior of the bag. This is one of the lightest and thinnest denier fabrics available. Compared to other ultralight down sleeping bags, this bag has a similar warmth-to-weight ratio as most of the lightest bags available.
According to Therm-a-Rest, the Hyperion 20F weighs approximately 1 pound 4 ounces. I weighed it myself and it is 1 pound 4.7 ounces.
The Hyperion 20F is an expensive sleeping bag. It is not the most expensive bag, even though it’s one of the lightest. But, it is far from the least expensive 20-degree sleeping bag available, either. For the price, you get one of the lightest full-featured summer-weight sleeping bags on the market.
This bag uses the most expensive 900-fill Nikwax treated down and premium ripstop materials. Some sleeping bags save costs and weight by eliminating the hood, much like a backpacking quilt, but the Hyperion has a full hood with a drawstring and draft collar.
As I mentioned above, there are certainly less expensive bags out there, but most aren’t as light. There are some comparably-weighted sleeping bags available that cost less, but those bags don’t have as many features as the Hyperion.
The Therm-a-Rest Hyperion 20F's ranges from $469.95 to $579.95.
Most sleeping bags come with a simple stuff sack, but this bag comes with a compression sack to compress the sleeping bag to the smallest possible size–about 6 by 8-inch cylinder.
The Hyperion uses 900-fill down insulation, which is the most packable kind of insulation available. Compared to synthetic insulation sleeping bags, this sleeping bag packs down significantly smaller. Even without a compression sack, this sleeping bag packs down much smaller than a comparably-rated synthetic insulated sleeping bag.
Compared to other down sleeping bags, this packs down very small with the compression sack. If you use a compression sack, you can pack other down bags to a similar size, but you’ll have to buy your own.
Remember, though, the more you compress a down sleeping bag, the longer it will take to regain its loft. If you don’t have to fully compress this sleeping bag to fit it in your backpack, I recommend you don’t. I prefer to simply pack my down sleeping bag into the bottom of my pack and compress it by stuffing the rest of my gear on top of the sleeping bag. I tried using the included compression sack with the Hyperion but ended up simply packing it into the bottom of my pack as I usually do.
Also, you should never use a compression sack to store a down sleeping bag long term. If you need to compress it fully to fit it into your setup, that’s okay, but you shouldn’t leave your sleeping bag fully compressed for days at a time.
The Hyperion uses 12.5 ounces of 900-fill down insulation in the standard-length bag. This bag is rated to 20 degrees Fahrenheit. But, you should really consider it a 32-degree bag because that’s what it’s rated to for comfort. This means that though you will survive temperatures below 32 degrees, you probably won’t feel warm enough to sleep through the night comfortably.
This is a 3-season sleeping bag. In winter or extremely cold conditions, you would need at least a sleeping bag liner to use this bag. I wouldn’t use this bag for winter conditions unless it was with a warm liner, second sleeping bag, or a quilt rated to around 40 degrees.
In the summer, I wouldn’t hesitate to use this sleeping bag without a tent. This bag will keep you warm enough in all but sub-freezing temperatures and the insulation provided by a tent is minimal.
This bag has a hood and a draft collar to help keep the warmth in. The draft collar extends along the length of the zipper to prevent cold air from getting in through the zipper as well. Also, this bag has 70% of its down on the top and sides of the bag, with only 30% on the bottom.
Compared to other bags with a similar fill weight, this bag is about as warm. No sleeping bag with the same amount of down will be significantly warmer than others. That being said, things like a hood and draft collar will make a sleeping bag significantly warmer than a bag that doesn’t have those things.
This is a narrow-cut mummy bag. If you only sleep on your back and don’t move while you sleep, this bag will be roomy enough for you. However, if you’re a side or stomach sleeper, or you toss and turn throughout the night, this bag will feel extremely constrictive.
When I say narrow, I mean narrow. I’m not very big. I wear size small clothing. But, this sleeping bag feels almost like a straight jacket. This is one of the least spacious sleeping bags I’ve ever slept in.
The regular-sized version of this bag that I tested is 80 inches long and has 57 inches of girth at the shoulders, and 49.5 inches of girth at the hips. The foot box girth is 43 inches, too.
As a side sleeper, I prefer wider-cut sleeping bags or quilts. Most sleeping bags that I’ve used have enough room for me to twist inside the bag while moving between sleeping positions. The Hyperion doesn’t allow this at all. The only way I can lay on my side with this bag is if I roll over from my back with the bag wrapped around me. But then I’m exposing the back of my body to the air with only 30% of the down in this sleeping bag to insulate it.
The narrow profile of this sleeping bag is how it is made to be so incredibly light while still having a 20-degree rating. Compared to other sleeping bags with a similar temperature rating, this bag will feel less spacious than most, but it will be lighter. If you can squeeze into this bag and be comfortable laying on your back all night, I say go for it. But if you toss and turn, or sleep on your side, don’t be taken in by the low weight.
The regular-sized version of the Therm-a-Rest Hyperion 20F is 80 inches long, with 57 inches of girth at the shoulders, 49.5 inches of girth at the hips, and 43 inches of girth at the foot box (as per my measurements).
DURABILITY & OUTER SHELL MATERIALS: 7/10
The Hyperion is about as durable as other ultralight sleeping bags on the market. It is made of a 10-denier ripstop nylon material, which is adequately durable as long as you’re careful. There are sleeping bags made with a thicker denier and, therefore, more durable material, but those bags are heavier.
Generally, the thicker the material, the more durable it will be. But thicker materials weigh more, too. In my experience, the 10-denier ripstop used on this sleeping bag is durable enough if you use it with a sleeping pad.
I have had issues with the SynergyLink™ Connector loops on a Therm-a-Rest Vesper quilt tearing out of this same material. But, Therm-a-Rest replaced the quilt under warranty when that happened. This hasn’t happened on this sleeping bag, and I don’t expect it to be an issue since these connectors aren’t as important as with a sleeping quilt.
Close-up of the Therm-a-Rest Hyperion 20F's outer shell made up of 10-denier ripstop nylon.
This sleeping bag is about as comfortable as any other narrow-cut mummy-style sleeping bag.
The interior lining material is relatively soft. It’s not as slippery as some ultralight sleeping bags, but it’s also not the softest lining material, either. If you’re used to ultralight sleeping bags, this bag’s lining material will feel similar.
Finding a sleeping bag that is both comfortable and lightweight is like looking for a unicorn. I don’t find this sleeping bag to be particularly comfortable, but I didn’t expect it to be, either. I’ve slept in much more comfortable sleeping bags, but most of those were significantly heavier than this one.
If comfort is your top concern, look for a wider-cut bag than this one so you can move around inside it. If weight is your top concern, then forget about comfort and get used to being comfortable enough in a bag like this, or get a down quilt.
This bag isn’t very versatile. It’s not warm enough to be used in the winter. And its half-length zipper means you can’t fully open it to ventilate during warmer temperatures, either.
This is one of the least versatile sleeping bags I’ve ever used. I would only want to use this bag for temperatures between 25 and 50 degrees Fahrenheit. That’s not much of a temperature range. You could probably push that range slightly on either end. But, that half-zipper and 32-degree comfort rating mean you can’t put it that much.
The Hyperion has one zipper with two zipper pulls. As already mentioned, the zipper goes half the length of the bag. The zipper pulls can be operated from both the inside and outside of the bag.
I manage to snag the zipper on the ultralight fabric pretty often when getting into this bag. There’s not enough space inside the bag to use two hands and zip this bag up completely, but that’s what you really need to do to prevent the zipper from getting snagged.
The zipper pulls don’t have a zipper plow to prevent them from catching on the ultralight material. And, I’ve caught the zipper on the draft tube that runs along the inside of the zipper most times when zipping myself into this bag. There is a stiff strip of material on both sides of the zipper to help prevent snags, but these don’t keep the draft tube out of the way.
Compared to other sleeping bag zippers, this is one of the worst systems I’ve used. The one redeeming aspect of this zipper is that it only goes halfway down the bag.
The Therm-a-Rest Hyperion 20F has one zipper with two zipper pulls that can be operated from the inside and outside of the bag.
DESIGN & FEATURES: 7/10
The Hyperion prioritizes weight savings over everything else. It has no stash pocket to store your phone and only one half-length zipper. This bag doesn’t do anything remarkable design-wise. Rather, its design relies on cutting down weight incrementally everywhere that is possible.
This bag has a hood, but it’s pretty small. It’s large enough to fit your head inside, but little else. I have a Sea To Summit Aeros Ultralight Pillow, and it doesn’t come close to fitting inside this hood. You’ll have to position a pillow under your head on the outside of the hood if you use one.
The hood is great for locking in warmth when it’s cold. And for that reason, the hood is definitely worth the weight. You could not take this sleeping bag down to 20 degrees if it didn’t have a hood.
There is a cinch cord and toggle to tighten the hood and collar. However, the cinch is only adjustable outside the bag, so you’ll have to adjust the tightness before zipping the bag shut.
Draft Collar: 8/10
There is a draft collar under the chin and along the hood of this bag. This is an extra baffle of down that seals your body’s warmth inside. This draft collar works well and extends down the zipper's length to prevent cold air from getting in through the zipper.
Foot Box: 8/10
This bag’s foot box is big enough to fit your feet but not too big that it loses thermal efficiency. However, it’s also not big enough to cross your ankles or move your feet around much.
Compression Sack: 8/10
This bag comes with a compression sack that allows you to compress it to an extraordinarily small size. It weighs 1.6 ounces and is helpful if your setup doesn’t have much space in terms of volume.
The compression sack of the Therm-a-Rest Hyperion 20F weighs 1.65 ounces as per my home scale.
SynergyLink™ Connectors: 5/10
These are removable ultralight straps that allow you to connect your sleeping bag to your sleeping pad. They’re designed to loop underneath your sleeping pad, then attach on both sides of the sleeping bag. They do seem to help, but they’re pretty difficult to attach. It took me a while to get used to these. They only weigh 0.2 ounces, so they’re probably worth carrying, though.