Sea-to-Summit Spark UL Sleeping Bag Review (5 Degrees F)

Sea-to-Summit Spark IV Sleeping Bag Review

The Sea-to-Summit Spark IV is a 5-degree (F) down ultralight sleeping bag that can be used for cold weather and winter and backpacking. It is insulated with 850 fill power goose down and comes with cold-weather features including a full-length snag-free zipper, a down-filled draft tube, and a draft collar.  At 31 oz, it’s also incredibly lightweight for a bag with its temperature rating, although it trades off weight for a much more confining fit. We prefer to have a little bit more space inside a cold-weather sleeping bag to accommodate extra clothing layers and store electronics and water inside to prevent them from freezing at night when temperatures drop close to zero.

Specs at a Glance

  • EN Temperature Rating Lower Limit: 5F
  • EN Temperature Rating Comfort (Women’s): 18F
  • Gender: Unisex
  • Weight: 1 lb 15 oz
  • Fill weight: 21.9 oz
  • Draft Collar: Yes
  • Draft Tube: Yes
  • Zipper: Left-side #3 YKK, two-way
  • Baffles: Box
  • Insulation: Ultra-Dry Down 850+ Loft
  • RDS-Certified: Yes
  • Storage sack: included
  • Compression sack: included
  • Exterior Shell Fabric: 10D Nylon; Interior Shell: 7D Nylon
  • Length: 6 feet, (6′ 4″ long size also available)
  • Dimensions: (Shoulder/Hip Feet) 61″/53″/37″
  • Recommended stuff sack size: 10L
The Spark IV has vertical baffles over the chest and horizontal ones over the legs to prevent down migration.
The Spark IV has vertical baffles over the chest and horizontal ones over the legs to prevent down migration.

The Sea-to-Summit Spark IV is a cold-weather sleeping bag insulated with 850 fill power down and rated to 5 degrees Fahrenheit. Weighing 31 oz, it’s has a classic mummy shape with a shallow mummy hood on top.  The down is distributed on both the front and back of the bag, with vertical baffles over the chest and horizontal ones from the knees down to keep the down from migrating and creating cold spots.

The Spark IV comes with all of the normal features you’d expect on a quality cold weather sleeping bag including a full-down-filled draft tube behind the full-length zippers, a down-filled draft collar that wraps around the front of your chest, and around the back of your shoulders, and neck. The fabric along the length of the zipper is reinforced with a stiffener to prevent snags, which can lead to tears in the fabric, and is a feature you really only find on premium sleeping bags.

While the Spark IV does have a down draft tube, it’s quite narrow.
While the Spark IV does have a down draft tube, it’s quite narrow.

A draft tube is designed to block drafts from entering a sleeping bag through the zipper and seal in your body heat. While it does its job on the Spark IV, we were surprised by the size and width of the draft tube which is one of the narrowest we’ve ever seen on a cold-weather bag.

The same can be said for the bag’s draft collar, which we also found small. In this case, we found it to be too shallow to form a tight seal around our neck and prevent heat from escaping. We addressed this by wearing a down sweater to help fill the gap.

The front half of the draft collar is too shallow to provide a tight seal over the chest.
The front half of the draft collar is too shallow to provide a tight seal over the chest.

The shoulder girth of the Spark IV is 61″ which is on the smaller end of the spectrum for a bag of this temperature rating. But that measurement is somewhat misleading as the top of the bag narrows considerably between the shoulders and hood. It’s so narrow and tight that it becomes very difficult to adjust the draft collar and hood controls one-handedly to get a snug and comfortable fit. The combination draft collar and hood control also proved difficult to manipulate with one hand in the dark.

The combined hood and draft collar control is difficult to use one handedly in the dark.
The combined hood and draft collar control is difficult to use one-handedly in the dark.

Once we got settled, the bag kept us warm throughout the night but it did take a while to “heat the bag up” inside. Still getting in and out for bathroom breaks at night was quite a production, since the collar and hood need to be released and readjusted every time you go outside. Even something as simple as zipping the bag is difficult because it tapers so much between the shoulders and the hood, putting considerable tension on the zipper, which is reinforced with a snap at the top.

We didn’t use the Spark IV in warmer weather because we were mainly interested in testing the bag’s cold-weather performance. The sleeping bag does have a two-way zipper, however, so you can vent the footbox or the upper part of the bag if you get too warm.

Comparable Cold-Weather Sleeping Bags

At 1 lb 15 oz, the 5-degree Sea-to-Spark IV is considerably lighter weight than the following 0-degree bags. While these bags are only rated for 5 degrees colder, the weight difference is largely due to the fact they have larger dimensions than the Spark IV, have larger sized draft tubes, draft collars, and hoods, or use heavier weight fabrics

Make / ModelWeightFill WeightFill PowerPrice
Big Agnes Starfire UL 02 lbs 14 oz31 oz850$580
Feathered Friends Ibis EX3 lbs 1 oz28.5 oz900$635
Feathered Friends Snowbunting EX 02 lbs 13 oz25.3 oz900$599
Marmot Lithium 02 lbs 9.5 oz27.8 oz800$499
NEMO Sonic 02 lbs 11 oz23.6 oz850$479
Sierra Designs UL Nitro 02 lbs 8 oz26 oz800$379
The North Face Inferno 02 lbs 14 oz29.3 oz800$519
Western Mountaineering Kodiak MF 02 lbs 12 oz30 oz850$675
Big Agnes Blackburn UL 02 lbs 12 oz23.5 oz850$500


The Sea-to-Summit Spark IV is an ultralight mummy-style sleeping bag rated at 5 degrees Fahrenheit. While it only weighs 31 oz, which is extraordinarily lightweight for a bag with its temperature rating, it is important to recognize that the weight savings comes with a significant reduction in internal space, ease of use, and comfort. While the Spark IV will probably work well for back sleepers with a smaller stature across the shoulders, we’d recommend that you try some larger dimensioned colder-weather sleeping bags, so you can make the weight/comfort-tradeoffs that align with your preferences and objectives. If you have to spend 12 hours in a sleeping bag on a cold night, some extra comfort is often worth its weight in gold.

Disclosure: The author received a sample bag from Sea-to-Summit for this review.

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Last updated: 2021-04-11 08:50:20


  1. Nice review. You don’t get to see too many reviews on Sea to Summit products. Although the specs of their sleeping bags tick so many boxes, it’s difficult to know how well they’d perform in real life. This review gives a good perspective beyond company’s stated specs.

  2. Hello philip i am looking for a ultra ultralight sleeping bag for 10 celcius / 10 degree which sleeping bag can you recommend for me ? i will use that for 40+ km thruhiking in southern italy and spain 10 c sleeping bag should be fine. my i want to carry 2.5-3 kg baseweight . thank you for answer love your fantastic blog i read them all <3

    • why don’t you do some of the legwork and identify a few options and I’ll let you know what I think about them, since I have no idea what your preferences or needs are. Thanks,

  3. I love SectionHiker and most of their reviews, but I respectfully disagree with several of the negative aspects of this review. I have a 2020/2021 model year Spark III 18F and find its warmth on par with other similarly rated bags, including Western Mountaineering (and that doesn’t happen often)! I thoroughly tested the Spark III’s draft collar and while it is indeed less bulky than most, it keeps me warm and more comfortable than many draft collars and puts more down fill on top of my body. I don’t need to adjust it when getting in/out of bag at night as this review states. Sea to Summit quotes a larger girth for the Spark III/IV bag and I find their measurements to be both accurate and comfortable whereas this review shows smaller girths. My favorite feature about the Spark, besides the weight, is the almost see through shell. This allows me to visually ensure the down is balance out before laying down for the evening. I know this review is for the Spark IV, but my experience with the Spark III’s similar features would lead me to buy a Spark IV with the utmost confidence.

    • Thanks for the comment Scott. I always value a second opinion. I took the girth measurements right off the S2S website, so I’m not sure what you’re referring too. My hypothesis is that the 61″ shoulder girth is correct (it is very hard to measure these things) but the top of the bag where the zipper terminates is much much narrower, leading to my claustrophobic experience using it.

      • It’s confusing. I think they have an error on the bottom / text portion of their website whereas the graphic slider at the top shows larger 64/56/39” girth for the Spark III and IV. This larger measurement seems accurate when comparing the bag to my Marmot Plasma 15. Either way, I do like my Spark III and wish all bag manufacturers would use more transparent liner material so you can get a visual on even down dispersement.

      • I just realized – my long bag is wider than the standard. That’s why it’s so much more comfy for me! :)

      • That would do it. Glad we got that resolved. :-)

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