Temperature Rating of Sea to Summit Spark SpIII Down:

EN13537: 2012 Comfort: -2°C / 29°F
EN13537: 2012 Comfort Limit: -8°C / 18°F
EN13537: 2012 Extreme: -26°C / -15°F

Comfort — the temperature at which a standard female can expect to sleep comfortably in a relaxed position.

Lower Limit(Comfort) — the temperature at which a standard male can sleep for eight hours in a curled position without waking.

Extreme — the minimum temperature at which a standard female can remain for six hours without risk of death from hypothermia (though frostbite is still possible)
------

Think how you feel coldness is as personal and individual as your preferred sleeping position.
To sleep comfortable and warm my first attention goes the mats. Sleeping bag and mats have to function together to provide warm and relaxed sleep if you suffer sometimes from back problems like me. Last item I use in coldness is a merino wool cap.

I ignore the extreme rating. I would only look at this if I will go mountain climbing in high altitudes or too regions known for strong wind. For me just a value of safetyness for extreme conditions.

I want to sleep relaxed and I use the layer principle to avoid starting sweating. Sweating means getting awake and opening the sleeping bag to regulate your personal climate feeling. So not a relaxed sleep if you have to do this a couple of times in the night.

Normally I sleep in T-shirt and short underwear.
If it`s getting cold I use long merino underwear. If it`s getting more cold I add a pure silk inliner to this which adds 5°C to 8°C. This liner I use if it is too warm for the sleeping bag.
Will the temperature fall more down and I got awake through freezing I cover my sleeping bag with a rescue blanket(Polyester/Aluminium Foil).
If I know I could get cold I always carry 3 packs of rescue blankets with me because they small, lightweigt and you can use them as a reflection layer between mats and your sleeping bag too.

I tried out some sleeping bags/mats and I got a lot of product advices. Finally I think everybody has to find out yourself what works best. I change my gear as my needs change, this mostly more by age than by adventure driven.

__________________