A Minimalist Sleeping Bag Quiver

Ultralight means everything should do double-duty. So why take your insulating clothes off to sleep? Why not use them in combination with your bag? Problem is, most bags on the market have 'efficient cuts' to minimize internal space. This cuts down on the volume of air your body must heat up to stay warm, requiring less energy to sleep comfortably, longer, but doesn't allow enough space inside the bag to wear much puffy clothing, if any. Stupid, if you're a minimalist or ultralighter.

I look at a sleeping bag's temperature rating as it's average temperature it can be comfortably used at, not the minimum. Here's why:

I wear nearly all my layers when sleeping, including puffy clothes. I don't bring extra clothes, just wear the ones I'd be bringing anyway. I do this for two reasons: one is because I hate sleeping in only baselayers if it's anything other than a 'warm' night in summer. It's nice getting out of my bag and staying warm because I'm properly clothed, not freezing when I get up cuz I'm in just a baselayer. This minimizes clothing changes and internal shelter space needed for clothes. The second reason is because if I compliment the bag with warm clothes, I can take a lighter bag. Since I'm sleeping in clothing I'm taking anyway, I'm saving weight, not adding, by my clothing pulling double-duty.

Have you ever slept outside at -35F inside a -40 down bag & vapor barrier liner bag, then gotten up to pee in the middle of the night? If you did it right, you were in just a light baselayer, since you were sleeping inside a VBL bag. And since you were inside a VBL bag, that means that light baselayer was damp with moisture and there you are, standing out under the clear, star & aurora-filled sky, in wet long underwear. Hmmm... doesn't seem very alluring, does it? I froze my ass off, even after getting back in the bag & VBL, which was plenty warm for the temps. VBL clothing and insulated garments over the top, combined with a lighter bag, would have been much better.

Or how about another example that's a little less extreme in temperature and of a more common weather condition?
You wake up at two in the morning, having to pee, and it's blowing 30 mph sideways rain on the crest of the Alaska Range. Snow is accumulating less than 500 feet above you, so it's around freezing at your camp. You had been hiking in it for a few hours and were kind of damp and fairly chilled, so you ate dinner and went straight to bed. There's two other people crammed in your three person silnylon pyramid and everything is soaked from hiking... not much space to don shell layers without soaking your friends in the process.
   -do you go out and pee in your baselayer because you brought your warm bag?
   -or do you go out in your insulated layers because you brought your lighter bag?
I did the latter and even though I got a little wet, I was completely dry (bag included) when I woke up in the morning, not from being chilled, either. I slept very well and plenty warm. It either would have been miserable going outside in those conditions in just a baselayer or I would have had to put on other clothes to stay dry/warm.

I have wide shoulders and even a 62" shoulder girth can feel moderately confining. If I wear my insulating layers inside my bag, I need at least a 64" shoulder girth, if not more. For summer temps, 64 or 65 would be perfect, as insulating layers above freezing are fairly thin. Below freezing, especially below zero Fahrenheit, I would need a 66" girth to accommodate all that down.
Some would argue that without the warm layers to take up that space, I'd be cold, as there'd be so much internal volume to heat up. I'm not worried, though, as I am a side-sleeper and like to cocoon myself when I sleep, pulling the covers snug up against my body. Any extra would get pulled and scrunched up near my body, eliminating the excessive volume. Being a side-sleeper also means Big Agnes style bags don't work for me, as the face opening stays up at all times.
Plus, I am only looking to gain 5-10º of warmth from the down & synthetic clothing, and not even a heavy down parka, just midweight down/synthetic layers. Wearing too thick of clothing inside might negatively insulate you - it could trap too much heat in the clothing and not warm the bag. Could you get cold from this? Maybe. I don't know... I've heard both arguments. Either way, opt for a light bag, get an additional 5-10º of warmth from midweight clothing layers, and have a very versatile clothing-sleeping system.
Just make sure the inside shoulder girth of your bag is large enough to accept however much volume the insulated clothing takes up. Don't compress the down in your bag from the inside out.

Vapor-barrier liners are necessary below +20F. The thickness of the sleeping bag and the temperature gradient between the inside of your bag and the outside air is thick/high enough that the warm moisture getting pushed out of your bag reaches the dew point before exiting the down and condenses, wetting out the down and freezing, if it's cold enough. Read more about VBLs and theory.

So, a 20º bag is smart because by adding a VBL at it's minimum comfort temperature (20), you extend it's range below it's temperature rating and keep the bag dry, exactly when you need to start using a VBL anyway. Add down clothing to the system, and you can take a 20º bag easily down to 5º, if not colder. (VBLs add about 10º of warmth and clothing 5-10º) While I said I don't like sleeping in just baselayers below freezing, I'd rather do that from 20-30º than try and make a 30 or 35º bag work down to 20º. VBLs are not necessary above 20, I'd much rather not use one if I don't have to, and clothing probably can't add more than 10º of warmth, reliably. That's a big word when you're far from home.

Could 20º be the ultimate sleeping bag rating? Comfortable from single digits up to the low thirties without shivering or sweating?

In conclusion, here's my idea of an extremely versatile sleeping bag quiver in which each bag is used for a 25-30º temperature range!
Also take note that it is composed of only three bags, which means I can afford to have all WM bags :)

30ºF & warmer
Western Mountaineering Caribou MF 35º
Block type down pockets keep the down in place and from shifting and leaving cold gaps. Wide girth accepts a puffy jacket to drop the rating to 30º or so. I'd go with a quilt for the weight savings, but I'm a sleeping bag guy. I like mummy bags, as they are mentally comforting and soothing since they seal up tight, which allows me to always get really good sleep. No VBL, so any additional warmth must come exclusively from clothing, which I'm only betting on 5º with the clothing I'd have in these temps.

5 to 30ºF
Western Mountaineering Alpinlite 20º long length
For 20-30º, just the bag & baselayer. Below it's 20º rating, down to about 10º, just add a VBL (it's needed anyway, so perfect!). Below 10º, add your insulated layers for temps down into the low single digits. Long length allows hoods on insulated clothing to be used and to wrap the bag up real tight around the face, down to blow-hole size.

-10 to 5ºF
Alpinlite & Caribou Combo
Sleep in the VBL & insulated clothing inside with Alpinlite with the Caribou draped over the top as a quilt. In Alaska, this is, strangely, a very rare temperature range. Usually, it's warmer or colder, since we pretty much just have summer & winter, with a three week spring and fall. If I were living in the Lower 48, I'd definitely opt for a 5 or 10º bag to use from -10 to 15 or 20 above, which would change the above bags.

-40 to -10ºF
Western Mountaineering Puma MF -25 long length
This ultra-warm bag can easily be taken down to or past -40 with VBL and insulated clothing. Big 66" shoulder girth accepts thicker clothes. Long length allows for booties & hooded clothing usage, plus wraps up & over head nicely.
I tried the Feathered Friends Peregrine and I liked everything but the hood/face opening. I couldn't get the hood to come down my forehead and the wide face opening is really challenging to cinch up to blow-hole size.