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Choosing a sleeping bag means balancing warmth, weight, packed size, and comfort. The temperature rating tells you the lowest temperature at which the bag should keep you comfortable, but your own sleep warmth and sleeping pad also matter. Down insulation is lighter and packs smaller, while synthetic insulation handles damp conditions better. For more on staying warm, see our guide to cold weather camping.
Sea to Summit makes bags for backpacking, car camping, and everything in between. Below we compare seven of their current models, each with an ISO temperature rating provided by Sea to Summit, so you can find the one that fits your camping style.
Every specification on this page comes from the manufacturer’s own product page, checked on 29 October 2026. We have not used these products ourselves, so the picks are based on published specifications only.
If you’re upgrading your sleep system, also check our comparison of sleeping pads and tips for sleeping comfortably.
How we chose these
- Temperature Rating The ISO Comfort rating gives a realistic idea of the bag’s warmth. Choose a bag rated for the coldest night you expect.
- Insulation Type Down is lighter and more compressible, while synthetic insulation stays warmer when wet.
- Weight and Packed Size Backpackers need a light, compact bag. Car campers can trade weight for more room and features.
- Shape and Roominess Mummy bags are thermally efficient but snug. Rectangular or semi-rectangular cuts allow more movement and side sleeping.
All specs below are for the Regular length and 30°F version of each bag, unless otherwise noted.
Comparison table
| Product | Temp Rating (ISO Comfort) | Weight | Insulation | Packed Volume | Shape |
|---|---|---|---|---|---|
| Circuit Down 30°F | 36°F | 28.9 oz (820 g) | 650+ fillpower duck down | 5.4 L | Semi-tapered rectangular |
| Spark Pro Down 30°F | 37°F | 21.8 oz (619 g) | 900+ fillpower goose down | 5.3 L | Mummy |
| Hamelin Synthetic 30°F | 37°F | 39.6 oz (1124 g) | Synthetic (Revive Loft) | 8.4 L | Contour |
| Boab Synthetic 30°F | 36°F | 49.1 oz (1391 g) | Synthetic (Revive Loft) | 10.2 L | Rectangular |
| Women’s Hamelin 30°F | 30°F | 51.0 oz (1447 g) | Synthetic (Revive Loft) | 12.7 L | Women’s Contour |
| Trek Down 30°F | 37°F | 39.5 oz (1119 g) | 650+ fillpower duck down | 8.2 L | Less-constrictive rectangular |
| Ascent Women’s 30°F | 30°F | 31.5 oz (893 g) | 750+ fillpower goose down | 6.8 L | Women’s mummy |
Specs are for the Regular length and 30°F temperature rating variant of each bag. Other sizes and ratings are available.
1. Sea to Summit Circuit Down Sleeping Bag: for versatile camping and travel
Sea to Summit describes the Circuit Down as a semi-tapered rectangular bag suited for backpacking, camping, and travel. It uses duck down rated at 650+ fillpower and treated to be water-resistant without PFAS, and has a wraparound zipper that lets you open it flat as a quilt. The shell and lining are made from recycled polyester.
- Fill weight: 411 g (Sea to Summit)
- Recommended user height: up to 6 ft 1 in (Sea to Summit)
Pros: Versatile zip system; recycled materials; roomy enough for side sleepers
Cons: Heavier than the Spark Pro; not as warm as the 15°F version
Check the Sea to Summit Circuit Down Sleeping Bag on Amazon
2. Sea to Summit Spark Pro Down Sleeping Bag: for ultralight backpacking
The Spark Pro is Sea to Summit’s ultralight down bag, using goose down rated at 900+ fillpower and treated to be water-resistant without PFAS. The 10D nylon shell and lining keep weight low, and the Free-Flow Zip system allows ventilation or use as a quilt. Sea to Summit says it can be zipped to a Women’s Spark for two-person use.
- Fill weight: 310 g (Sea to Summit)
- Packed size: 13 x 8 x 5 in (Sea to Summit)
Pros: Very light for a 30°F bag; high-loft down; compact packed size
Cons: Mummy shape is snugger than the semi-rectangular Circuit; less room for side sleepers
Check the Sea to Summit Spark Pro Down Sleeping Bag on Amazon
3. Sea to Summit Hamelin Synthetic Sleeping Bag: for wet-weather backpacking
The Hamelin is a synthetic bag built for wet conditions, according to Sea to Summit. It uses Revive Loft insulation made from recycled polyester and has a weight-efficient Contour shape. The Free-Flow full-length zipper and short side zipper offer customizable ventilation.
- Fill weight: 710 g (Sea to Summit)
- Best for light trekking, backpacking, paddle touring, and car camping (Sea to Summit)
Pros: Synthetic insulation handles dampness; roomy Contour shape; dual zippers for ventilation
Cons: Heavier and bulkier than down bags; packed volume 8.4 L
Check the Sea to Summit Hamelin Synthetic Sleeping Bag on Amazon
4. Sea to Summit Boab Synthetic Sleeping Bag: for car camping and casual trips
Sea to Summit positions the Boab as a roomy synthetic bag for car camping, travel, and overnight treks. It has a full-length zipper with dual sliders and can be opened flat as a quilt. The insulation and fabrics are 100% post-consumer recycled polyester.
- Fill weight: 850 g (Sea to Summit)
- Length: 85 in (216 cm) (Sea to Summit)
Pros: Very roomy; can open flat; recycled materials
Cons: Heaviest of the 30°F bags here; large packed size
Check the Sea to Summit Boab Synthetic Sleeping Bag on Amazon
5. Sea to Summit Hamelin Women’s Synthetic Sleeping Bag: for women backpacking in damp conditions
The Women’s Hamelin is a women’s-specific synthetic bag with extra insulation in a Contour shape that Sea to Summit says allows natural sleeping positions. It has the same Free-Flow dual zipper system as the unisex Hamelin and uses recycled Revive Loft insulation.
- Fill weight: 1070 g (Sea to Summit)
- Recommended user height: up to 170 cm (66.93 in) (Sea to Summit)
Pros: Women’s-specific fit with more insulation; good for damp conditions
Cons: Heavier than the unisex Hamelin; shorter max user height
Check the Sea to Summit Hamelin Women's Synthetic Sleeping Bag on Amazon
6. Sea to Summit Trek Down Sleeping Bag: for camp comfort and versatility
The Trek Down is designed for versatility, with a less-constrictive shape and a Free-Flow Zip system that lets you open the bag flat or wear it around camp, according to Sea to Summit. It uses duck down rated at 650+ fillpower and treated to be water-resistant without PFAS, with a polyester shell.
- Fill weight: 380 g (Sea to Summit)
- Can be worn around camp using shoulder snaps (Sea to Summit)
Pros: Versatile zip and wear system; comfortable shape
Cons: Heavier than the Spark Pro; lower fill power than some down bags
Check the Sea to Summit Trek Down Sleeping Bag on Amazon
7. Sea to Summit Ascent Women’s Down Sleeping Bag: for women’s three-season backcountry
The Ascent Women’s is a three-season down bag with 750+ fillpower goose down and a women’s-specific cut, says Sea to Summit. The Free-Flow Zip system allows ventilation and quilt use, and the bag can be worn at camp. It has a technical hood and draft-tube protected zippers.
- Fill weight: 520 g (Sea to Summit)
- Recommended user height: up to 5 ft 7 in (Sea to Summit)
Pros: High fill power down; women’s-specific warmth; versatile zip
Cons: Shorter max user height; mummy cut may feel snug
Check the Sea to Summit Ascent Women's Down Sleeping Bag on Amazon
Which one fits you
| Your situation | Look at | Why |
|---|---|---|
| You want the lightest possible backpacking bag | Spark Pro Down | It uses 900+ fillpower down and weighs just 21.8 oz, with a compact packed size. |
| You camp in wet or humid conditions | Hamelin or Women’s Hamelin | Synthetic insulation stays warm when damp, and the Contour shape allows movement. |
| You need a roomy bag for car camping | Boab Synthetic | It has a wide rectangular cut and can open flat as a quilt. |
| You want a women’s-specific down bag for three-season trips | Ascent Women’s Down | It has extra insulation and a shape designed for women, with a versatile zip system. |
Practical tips
- Consider the ISO Comfort rating rather than the lower limit when comparing warmth, as it reflects the temperature at which a typical sleeper stays comfortable.
- Down bags are lighter and more compressible, but synthetic bags are a better choice if you expect wet weather or high humidity.
- A bag with a full-length zipper and the ability to open flat gives you more options for ventilation on warmer nights.
- Use a sleeping bag liner to add warmth and keep your bag clean, which can extend its life.
FAQ
What is the difference between down and synthetic insulation?
Sea to Summit uses down in bags like the Spark Pro and Trek for light weight and compressibility, while synthetic bags like the Hamelin and Boab use Revive Loft insulation that performs better in damp conditions.
Can I zip two Sea to Summit sleeping bags together?
Sea to Summit says the Spark Pro can be zipped to a Women’s Spark for two-person use. For other models, check the product details for compatibility.
How do I choose the right temperature rating?
Look at the ISO Comfort rating on the bag. Sea to Summit provides both ISO Lower and ISO Comfort ratings; the Comfort rating is a better guide for most campers.
Sources
Specifications from each manufacturer’s product page, checked 29 October 2026: Sea to Summit Circuit Down Sleeping Bag, Sea to Summit Spark Pro Down Sleeping Bag, Sea to Summit Hamelin Synthetic Sleeping Bag, Sea to Summit Boab Synthetic Sleeping Bag, Sea to Summit Hamelin Women's Synthetic Sleeping Bag, Sea to Summit Trek Down Sleeping Bag, Sea to Summit Ascent Women's Down Sleeping Bag. Makers can change specifications; check the product page before you buy.
