Sea To Summit Spark IV Down Sleeping Bag Review

Last Updated on 10/10/2023


In this Sea To Summit Spark IV down sleeping bag review, I share my recent experiences and thoughts on one of the most lightweight and compact 4 season sleeping bags in the world. You will learn what makes this bag such a good choice for thru-hikers as well as how it compares to other similar sleeping bags.

Sea To Summit Spark IV Down Sleeping Bag Review

In a hurry? Here’s the short review:

The Sea To Summit Spark sleeping bag range is one of the best in the world. There are five variations, including a liner, that all follow the same design but with different temperature ratings and fill weights. The one we have been testing recently is the Spark IV, which is the warmest of the five, with a temperature rating of up to -15 degrees Centigrade.

Initial thoughts were very good. It arrives in the storage sack instead of the stuff sack, so the down is partially lofted when it arrives – just as I would expect. As soon as you open it up, it begins to grow as the 850+ FP down expands to its full potential. It is nice and wide, which is a nice surprise because some lightweight sleeping bags can be a little too tight due to the mission to save weight.

The material itself feels like silk and is so thin you can see the down feathers inside. That doesn’t worry me, though, as it feels more than strong enough, and the construction quality is very high. All the seams are tight, and it feels like every inch of it has been inspected and engineered to work in harmony.

At full loft (fully expanded), it is amazing to climb into as it feels like some kind of insulation cloud that weighs so little. The zip is, without a doubt, the best I have ever used. Unlike ALL my other sleeping bags, the zip NEVER gets caught on the material or snags when you try to zip it up. More on this later.

Overall, my first impressions couldn’t have been any better, and I still don’t have anything bad to say about it.

Sea to Summit Spark Ultralight Down Sleeping Bag, 40-Degree, Regular


Benefits of the Sea To Summit Spark IV Down Sleeping Bags

Some of the main reasons to get this sleeping bag are:

  • Ultra lightweight
  • Ultra compact with two types of storage sack
  • Very warm
  • Super soft and comfortable
  • Expertly designed for a good night’s sleep and ease of use
  • Zip never snags and can be operated entirely with one hand
  • Available in multiple sizes and temperature ratings
  • High-quality materials and construction

Downsides of the Sea To Summit Spark IV Down Sleeping Bags

Some reasons I found that might not make this particular sleeping bag suitable for you are:

  • It is a high-end sleeping bag with a fairly high price
  • The material is very thin

Features and Specifications

Here are the features of the Sea To Summit Spark IV Down Sleeping Bag:

  • PACK SIZE: 8 x 11 in / 28 x 20 cm / 6.6 Liters
  • WEIGHT: 31 oz / 880 g
  • FILL WEIGHT: 21.9 oz / 620 g
  • SIZE/SHAPE: Regular Mummy
  • INSULATION: Ultra Dry 850 FP Down Insulation
  • MATERIALS: 10D Nylon Shell With 7D liner Fabric
  • Box Baffle Construction

Sea To Summit Spark IV open sleeping bag

Warmth and Temperature Rating

The Sea To Summit Spark IV is incredibly warm for a sleeping bag that weighs under 900 grams. It has a maximum temperature rating of 5°F / -15°C, so for most of the year, it will be more than warm enough for anyone who is a cold sleeper. I haven’t tested this sleeping bag past freezing temperatures yet, but I will keep this updated so that next year, I will have used it for a full four seasons.

The 850+ down is rated for 3.5 seasons, which means that you can use it all year round, and only in the most extreme conditions might you need some extra warmth like a liner. Where I live, winter temperatures rarely fall below -15°C, so I would consider it to be a full four-season sleeping bag.

Because the liner and high loft goose down make this bag feel so luxurious, it seems to warm up in an instant and maintain a steady heat all night long. There are no cold patches, and if it is a warm night, you can open the full-length zip to let some heat out.

Sea To Summit Spark IV sleeping bag top view

Comfort and Fit

What I really like about the fit of this sleeping bag is its width, which is 59 inches (circumference) at its max width or about 30 inches across. The mummy shape tapers towards the feet, but you still have plenty of space to lay with your legs apart. It is a dream to slip into, zips up with no snags, and instantly starts to warm up. You can see that it even overlaps the edges of the Ether Light XT Sleeping Pad.

The material liner is a 7D nylon, which is very silklike and feels like the inside of an expensive dinner jacket. This makes moving around almost frictionless so that you never feel tangled up in loose fabric. It’s nice and quiet, and as soon as you settle and down expands fully, it is very easy to drift off to sleep.

The toe box is amazing. When you point your feet upwards, you can see how it is designed not to stretch but to move with your feet instead, which again helps to avoid any cold spots. There seems to be extra down stuffed around the footbox area as well as around the hood, which keeps your feet warm and your head cushioned and cozy.

Such a comfortable sleeping bag.

SPark 4 sleeping bag sea to summit review

Weight and Pack Size

One of the main reasons I wanted this sleeping bag was for its weight and pack size to warmth-ratio. Meaning I wanted the most lightweight and compact sleeping bag in comparison to its warmth rating. The Sea To Summit Spark IV is just that, and you can see where it ranked in our smallest sleeping bag guide here.

It weighs 31 ounces and has about 22 ounces of down insulation stuffed into all of the individual baffles. It will pack down into a tiny 8 x 11-inch stuff sack for short-term packability and also includes a larger storage sack for protecting the loft of the down when not in use. Both of these specifications are made more impressive in that it will keep you warm down to a freezing -15°C in winter.

If you need an ultra-warm sleeping bag but prioritize pack size and weight too, then this is the number one sleeping bag I’d recommend.

Sea To Summit Spark IV sleeping bag materials

Materials and Construction

I am very happy to report the craftsmanship and quality of construction surpassed my expectations. The shell and lining are incredibly soft, and everything holds together very well. All the stitches are tight, and it feels very well made. Even though the shell and lining are very thin, they are more than durable enough for the demands of camping.

The Sea To Summit Spark IV is set up to handle moisture better than your average down sleeping bag. With hydrophobic ULTRA-DRY down that absorbs 90% less water than untreated down and a water-repellent treatment on the outer shell, you don’t have to worry about it getting wet from damp conditions too much.

Something I wasn’t expecting is the certificate of testing the sleeping bag has undergone before being shipped. This comes in a fancy threaded envelope and shows that it is 97% pure down feathers, and 91% of those come from geese. It also shows that the loft is higher than the stated 850 and that it comes from RDS-certified sources.

Sea To Summit Spark IV sleeping bag hood

Hood Design

The hood is comfortable and spacious when left fully open but can be fully sealed up around your face with an easy-to-pull drawcord. It features thick baffles around the edge as well as around the neck, which helps to trap body heat. It does a great job of keeping your head warm and comfortable and doesn’t get in the way if you constantly toss and turn in your sleep. 

The hood is large enough to fit an Aeros pillow inside or some soft clothing substitute if you don’t have a pillow. You don’t need to tighten it very much before the face hole starts to get smaller, and I never struggle to find the cord, which is located at the top right side. A feature I wish all sleeping bags had, including this one, is a detachable mosquito net cover to help keep the bugs away from your face when sleeping outdoors.

Sea To Summit Spark IV baffle and zip

Baffles and Zips

The different variations of this sleeping bag have different baffle designs; here is a list:

  • 10°C and 5°C have sewn-through construction
  • -2°C has baffles in the torso and sewn through in the leg area
  • -8°C and -15°C are fully baffled

The Sea To Summit Spark IV -15°C is fully baffled, so each tube-like segment of the sleeping bag is completely sealed off from the ones next to it. This is the best way to ensure that the down doesn’t clump together and remains evenly distributed to eliminate the chance of getting cold spots where all the down has shifted away from.

The vertical torso baffles on the top half are designed to prevent down from shifting as you roll in your sleep, whereas on the bottom half are designed to focus more towards your feet, which are one of the first places to get cold.

The zip is a number 5 YKK which is one of the best I have tested. This is aided by the internal zip baffle having a rigid area next to the zip to prevent the material from ever getting snagged or caught. I never realized how much I would appreciate this until I got this sleeping bag.

Sea To Summit Spark IV

Final Verdict

The Sea To Summit Spark IV down sleeping bag is my new favorite cold-weather sleeping bag. It has the best warmth-to-weight ratio of any sleeping bag and will also pack down to be one of the smallest too. This makes it perfect for lightweight hikers and backpackers who are willing to pay a little extra for the best in the world.

So far, I’ve tested the sleeping bag at the tail end of summer and early fall; I will update this review after winter to let you know how it goes when it gets really cold.



We hope you enjoyed reading this Sea To Summit Spark IV Down Sleeping Bag Review and found our insights both useful and fun to read. Let us know your thoughts in the comments.

Gear Assistant
Gear Assistant

This article has been written and/or edited by Andrew N. 20+ years of hiking, mountaineering, and camping experience, with access to all the latest outdoor gear.

Gear Assistant