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Sea-to-Summit Spark 1 Sleeping Bag Review (40-degree)

Sea-to-Summit Spark Sp1 40 Sleeping Bag Review

The Sea-to-Summit Spark 1 (40-degree) is an ultralight down sleeping bag that weighs 12 ounces (340g) and is insulated with 850-fill power waterproof down. The Spark 1 has a sewn-thru construction and a one-third-length zipper to save weight. When stuffed the Spark 1 packs down to the size of a baseball (1.5L) making it perfect for trips where pack space is at a premium. Its unbelievably low weight will appeal to anyone trying to shave ounces or grams from their pack weight.

RELATED: 10 Best Down Sleeping Bags for Backpacking

Specs at a Glance

  • Gender: Unisex
  • Weight: 12 oz (Size regular, Length 72″)
  • Actual Weight: 12.5 oz on the SectionHiker digital scale
  • Fill Weight: 6.3 oz
  • Sizes: regular,
  • Type: Mummy
  • Packed Size: 1.5L
  • EN Limit Temperature Rating: 40F (5C)
  • EN Comfort Temperature Rating: 48F (8.9C)
  • Construction: Sewn-thru
  • Girth/Hips/Footbox (size regular): 59″, 51″, 35″
  • Insulation: 850 Fill Goose Waterproof Down RDS
  • Zipper: One-third zipper, 2-way
  • Zipper Side: Left
  • Draft Tube: No
  • Shell/Liner Fabric: 10D Nylon RipStop with DWR / 7D Nylon RipStop
  • Stuff Sack: Compression sack included and a cloth storage bag
  • Visit Sea-to-Summit for complete Specs

Ultralight Design Features

At just 12 oz, how did Sea-to-Summit 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 59″ shoulder girth (size regular) but there isn’t a whole lot of extra room for me. The footbox is rather spacious, however, and my feet can easily splay apart comfortably.

Next, Sea-to-Summit uses very lightweight fabrics on the Spark 1 including a silky 7D nylon liner and an ultralight 10D exterior nylon shell. Both of these fabrics are PFC-free.

Finally, the Spark 1’s zipper is only 1/3 length providing extra weight savings. Because it’s a warm-weather bag, it doesn’t have a zipper draft tube or a chest draft tube, which saves additional weight. The zipper is two-way and has a snap at the top so you can unzip and vent the torso if you get too warm. The snap also takes any tension off the zipper, adding to its longevity. However, the zipper does not have any anti-sag taping along its length, again to save weight, so you need to be careful not to have it jam or potentially tear the lining and shell fabric when opening or closing it.

The baffles are oriented vertically over the chest and horizontally over the legs.
The baffles are oriented vertically over the chest and horizontally over the legs.


The Spark 1 is insulated with 850 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. The waterproofing makes the down dry faster if it gets damp or wet

The down insulation is oriented in vertical baffles over the hips and chest and in horizontal baffles over the legs and feet to help prevent down shift and keep the down where you want it. The baffle construction is sewn-thru, rather than being box-baffled to save weight and keep costs lower. In sewn-thru construction, the top and bottom fabric shells are sewn together to create tubes to hold the down in. While this prevents downshift, it can create cold spots at the seams.

The Spark 1 has a 1/3 length zipper to save weight.
The Spark 1 has a 1/3 length zipper to save weight.

While the Spark 1 is rated for 40 degrees, I’ve found it sleeps a little cold at 50 degrees, even when used with a suitable sleeping pad and long underwear. It’s still a viable option for warmer weather use, but I suggest you shakedown its comfort rating before counting on it close to its comfort limit.

The hood is flat and shallow, making it easy to sleep on your side, while still providing some coverage.
The hood is flat and shallow, making it easy to sleep on your side, while still providing some coverage.

Mummy Hood

The Spark 1’s mummy hood is pretty standard with a right-side elastic cord that lets you cinch the hood closed. It doesn’t have space for a pillow, but it is wide enough that you can comfortably roll over onto your side while wearing it. The 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 hood is also quite unobtrusive if you decide you want to unzip up the sleeping bag and use it more like a quilt.


The Sea-to-Summit Spark 1 (40) is an ultralight sleeping bag that weighs 12 ounces and is optimized for backpacking trips where space and weight are at a premium. It is by far one of the lightest sleeping bags or backpacking quilts available today.  For example, a custom Enlightened Equipment Revelation Quilt with comparable specs (fill power, fabric weight) weighs 3 ounces more. The advantage of using a sleeping bag like the Spark 1 over using a backpacking quilt, besides the obvious low weight and packability, is that you’ll have a draft-free experience without needing a bivy sack because it’s completely enclosed. If you prefer a sleeping bag over a quilt and want an ultralight sleeping bag, the Spark 1 is a viable option worth considering.

Disclosure: Sea-to-Summit donated a sleeping bag for review.

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  1. “Packs down to the size of a baseball” is what caught my eye, along with the weight. What are your thoughts on keeping this in a daypack as emergency insulation for unexpected overnights for 3 season day hiking? Same weight as a 50F Apex quilt but less than half the packed size, warmer, and down will do better than synthetic staying compressed all the time.

  2. I wouldn’t get too excited about the “size of a baseball” thing. If it packs to 1.5 liters and no smaller, that’s over seven times the volume of a baseball!

  3. Great review and nice alternative to other bag options. I have and use the western mountaineering everlite overbag for similar environments. Not promoting one over the other but I am with the full length zipper and ability to use over my traditional sleeping bag to provide additional warmth to the bag or use it separately as a warm weather bag. As always, I really enjoy and always learn from your reviews.

  4. Good review, you came to the same conclusion I did, that the 40 degree rating was quite a bit off. Because of that I returned my Spark 1 and went with another sleeping bag on your “10 Best Ultralight Sleeping Bags of 2023”, the Western Mountaineering FlyLite. For just 2.25 oz more for the 6’ size you get a bag that is not sewn-through, has a draft flap, has an insulated top collar, has 1.65 oz more fill, has a foot girth of 39” vs 35”, and is rated to 36 degrees (which I have found to be pretty accurate). At only 14.25 oz it’s will worth the consideration.

    • Recently purchased this bag and have yet to take it into the backcountry but tested it in my backyard sleeping in my quarterdome sl 1 with outside overnight air temperatures ranging from 37-40F.. Slept comfortably warm in a merino hoody and farpointe alpha direct pants…woke up around 3am and felt a bit chilled (felt like it was through the top of the bag where there is no draft collar… put on my puffy and slept fine until waking up at 7.. wouldn’t say it was toasty warm but I was never uncomfortably chilled or cold.. probably wouldn’t use this bag if night time temps were less than 40F but feels like you could make it work with proper sleeping clothing for added warmtu.. purchased it primarily for summer use when overnight temps typically are 45-55F.. this bag seems perfect for that especially if trying to keep your base weight as low as possible.

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