I got this crazy idea. Sleeping bags are more effective at keeping you warm if they fit properly - loose enough not to collapse the loft but tight enough not to have unnecessary air space. So why not just do away with a sleeping bag altogether and use a coat and ski pants instead? They're close fitting, warm, comfortable, and potentially even lighter and less expensive. For nights when it's a little colder, you could use a vapor barrier layer over your base layer so as not to wet out your insulation. If you sweat too much and get your base layer wet, the VBL would still hold the heat in, and I bet this setup with an extra dry base layer (for day) would still be lighter and less bulky than carrying warm clothes for camp and a sleeping bag. Of course, I know it wouldn't work for extreme conditions, probably no colder than 20F at the lowest (just guessing), but for moderate to warm climates, do you think this would work as a comfortable three season setup?

Or, if you want to allow your legs to share warmth (like how mittens are warmer than gloves), you could use a half length sleeping bag. Just thinking out loud here.

Hazy recollection of down pants that could join the legs as a quasi-bag using zippers or somesuch. Combined with a thick parka and maybe a simple bivy sack you have a kind of sleep system--more for climbing than leisurely camping, though.

"Elephant's foot" half-length sleeping bags that pair with a parka have always been around, although their popularity ebbs and flows. For anybody who owns a fat parka, that's a good option for 3-season sleep system with one-and-a-half, if not dual-use.

Then there are the bags and quilts that can be repurposed as down camp clothing. Jacks r Better and Feathered Friends come to mind.

Cheers,

At least for me, a VBL is like a Turkish bath until the temp gets down close to freezing. I know some who can't tolerate a VBL at much colder temps. For me, a VBL is not so much for extra warmth as for keeping body moisture out of my sleeping bag and/or puffy clothing insulation on below-freezing nights.

I won't go out without a combination of insulation that will keep me comfy at close to the record cold temps for the area/time of year. Being too lazy to look this up for every trip, I just assume 10*F for high alpine areas. I've needed this for Alpine larch peeping trips in the high northern Washington Cascades in early October--which is as late in the year as I backpack due to my dislike of being stuck long nights in the tent. Also for cold nights in the Rockies, which are much higher altitude.

I suspect that a warm sleeping bag is the most efficient method of insulating. It traps your body heat the most efficiently for its weight. As you said, the gloves (clothing pieces that keep your arms and legs separate) vs. mittens (the sleeping bag).

I've tried it to reduce weight and bulk, sleeping in a jacket and fleece pants, but the results were poor for me. I was warm enough, but it lacked the comfort and perhaps, security, a sleeping bag or quilt provides. That might be admitting to some emotional and psychological issues, but hey, it's my hike. If I was really enthralled with the idea, I'd at least use a light quilt. I didn't like the fact you had a much harder time adjusting sleeping temps. Large temp. swings are easier handled with a bag or quilt for me. I think I'd sleep better with a just 2/3 length quilt over my torso than using the elephant's foot/jacket combination.

The nylon that encloses the down weighs more than the down. Clothing has proportionally much more nylon for the same amount of down, so will be heavier than an equivalent weight of down enclosed in a sleeping bag. Ski pants are a heavy alternative - since they are made to withstand lots of abuse and thus have hefty outer material. But the main reason I would not use clothing as sleeping insulation, is that clothing can get wet and it would leave you in a pickle. My sleeping bag is packed away, in a waterproof stuff sack (dry sack)and I do not take it out until it gets inside the tent.

You can get good 1.5-2 pound down bags nowadays, and be plenty warm in most 3 season conditions. I really see no advantage of the clothing system. Good quality down clothes also would probably cost more than a sleeping bag.

True on all counts, I'm sure, but that's if you're insisting on the lightest of everything (down). What if you're going for inexpensive (synthetic), like me? My coat's no parka, but it'd probably take me down to 50F nights or colder, and it cost me less than $20, tax included.

Here is a case of regional differences. If your 3-season requirement is only 50 degrees, and if you are in a very rainy humid climate, then synthetics are good, particularly if cost is more of an issue than bulk and weight. But I still maintain that a synthetic bag will also be warmer for the total weight than the equivalent synthetic clothing.

Your situation is quite different from mine, where my 3-season temperatures can go down to well below freezing. In fact my 1-season (summer) temperatures can go down to 20 degrees! Plus in the Sierra, only considering summer, we have a vast range of conditions from 90-degree day temperatures in the foothills (and Yosemite valley) to 20 at night at 12,000 feet. Plus my typical day includes 3000-5000 feet elevation gain so I really have to keep pack weight down.

Sleeping bags weigh less than the equivalent warmth clothing. Clothes provide coverage and insulation in a lot of nooks and crannies. When comparing equivalent insulation you should be careful. Just because a jacket you wear is warm at a particular temp does not mean it will keep you warm sleeping at that temperature. Your metabolism drops quite a bit while sleeping compared to moving around. You need much more insulation.

To me, your eliminating the last defense against misread weather forecasts or surprise wind/rain events than make it seem much colder than it is. If you're tired from the trail, you're body will have a great more difficulty maintaining core temps in the wee hours. I was much more discomfort tolerant when I was younger, so my choice now is to almost guaranty comfort and safety as much as I can. A sleeping bag/quilt is part of achieving that; sleeping in my jacket was something I did at the base of climbs or on quick trips when I was young. If you think you can have a weight and financial savings by sleeping in your jacket, try it at home first and see how it goes.

IMO
the coldest point in the day cycle is about 4am. This also hits during the time we haven't exercised for the last 5 hours. It's when we need the most insulation. If you bring enough clothes to be warm during this period, you've brought too much clothes. a sleeping bag has more insulation/gram than a clothing format that doesn't encompass your whole body.