The Therm-a-Rest Hyperion 20 is an ultralight down sleeping bag designed for back sleepers that weighs 20 ounces (1 lb 4 oz) and is insulated with 900 fill power waterproof down. It has a box-baffled construction to eliminate cold spots and a half-length zipper to save weight. When stuffed the Hyperion 20 packs down to just 3L making it perfect for trips where pack space is at a premium. The insulation is zoned with 70% on top and 30%, leveraging your sleeping pad insulation for warmth. What’s the catch? It has a tapered cut with limited girth, but if you can fit inside comfortably, it’s a sweet ultralight sleeping bag.
Specs at a Glance
- Gender: Unisex
- Weight: 20 oz (1 lb 4 oz) – Size Medium, Length 72″
- Fill Weight: 12.5 oz
- Sizes: S, M, L
- Type: Mummy
- Packed Size: 3L
- EN Comfort Temperature Rating: 32F (-6C)
- EN Limit Temperature Rating: 20F (-23C)
- Construction: Box Baffled
- Girth/Hips/Footbox (size regular): 57″, 49.5″, 43″
- Insulation: 900 Fill Goose Nikwax Hydrophobic Down RDS
- Zipper: Half-Zipper
- Zipper Side: Left
- Draft Tube: Yes
- Shell/Liner Fabric: 10D Nylon RipStop with DWR / 10D Nylon RipStop
- Stuff Sack: Compression sack included and a cloth storage bag
- Visit Therm-a-Rest for complete Specs
The Hyperion 20 sleeping bag is a backpacking and mountaineering mummy-style sleeping bag designed for back sleepers. I can’t emphasize the latter more. The insulation is distributed in a zonal fashion with 70% on top and 30% underneath you, leveraging the cold resistance of your sleeping pad to keep you warm. If you roll over onto your side, you’ll sleep much colder than if you sleep on your back with the bulk of the goose-down insulation on top of your torso. To prevent this, the Hyperion 20 comes with optional sleeping pad straps that connect to loops on the back of the bag. These act similarly to quilt-pad attachment straps to keep you on your sleeping pad at night.
Ultralight Design Features
At just 20 oz, how did Therm-a-Rest make this sleeping bag so lightweight?
First off, the sleeping bag is quite tapered and is designed to fit you without a lot of extra internal space. I can fit into its 57″ shoulder girth (size regular) but there isn’t a whole lot of extra room for me inside from my waist up. The footbox is rather spacious, however, and my feet can easily splay apart comfortably.
The Hyperion is insulated with 900 fill power Nikwax waterproof goose down which is premium stuff in terms of warmth-to-weight ratio. Higher fill power down like this provides more warmth by weight than lower fill power down and is used to save weight.
While the Hyperion 20 has a two-way zipper, it’s only half-length, so it ends at my knees: this saves weight by eliminating the need for a longer zipper, draft tube, and anti-snag taping, which can be found on the upper half of the zipper. The two-way zipper makes it possible to vent the bag to a certain extent, although not in the footbox. If you get habitually hot feet or you need to camp in very hot/humid weather, I’d get a different sleeping bag with a full-length zipper instead.
The lightweight 10d ripstop nylon fabric on the Hyperion is somewhat delicate and you’ll want to treat it gently to prevent it from tearing. The anti-snag tape along the zipper draft tube helps prevent rips, but you’ll want to be careful with it nevertheless.
The hood is a pretty standard mummy hood with a right-side control to let you cinch the hood closed. It doesn’t have space for a pillow or anything, but it will move with you if you roll over like a jacket. A snap at the top of the side zipper helps seal the hood on the left side and maintain its shape even if the side zipper is not fully closed.
The Therm-a-Rest Hyperion 20 is an ultralight sleeping bag designed for back sleepers who prefer the draft-free performance of a sleeping bag over a backpacking quilt. Weighing 1 lb 4 oz, it is optimized for backpacking trips where space and weight are a premium. It is by far one of the lightest sleeping available today but it’s a snug fit, harder to vent, and best used in cooler temperatures rather than hot and humid ones. The advantage of using a sleeping bag like this over using a backpacking quilt, besides the obvious low weight and packability, is that you’ll have a draft-free experience because it’s completely enclosed. The disadvantage is that it has a narrower temperature range and you can’t side sleep with it. But if you prefer a sleeping bag, you back sleep, and need a bag for cooler, shoulder season (spring and autumn) weather, it’s a high-performance choice. A 32-degree model is also available.
Disclosure: Therm-a-Rest donated a sleeping bag for review.SectionHiker is reader-supported. We only make money if you purchase a product through our affiliate links. Help us continue to test and write unsponsored and independent gear reviews, beginner FAQs, and free hiking guides.