1. How cold can you sleep comfortably naked? I mean without the sleeping bag or any covers. Apparently you can get aclimatized to it. I'm not sure what the practical value is in this except to get to know more about heat loss from the human body and skin temperature. It would depend on such things as air temperature, humidity, rain, raidant heat loss or gain from surrounding object, and what surface you are sleeping on. Let's say there is a tarp overhead, or you are in your living room. I've tried this at home and found that you have to give yourself some time to get used to the cooler skin temperature, but I have a feeling it might actually be healthy.
2. How cold can you sleep naked except for an 8oz ground pad, a 20oz wool blanket, and a 4oz nylon shell? Let's say there is a tarp overhead.
3. How cold can you sleep naked except for an 8oz ground pad, a 20oz wool blanket, a 4oz nylon shell, plus lets say 16oz of summer clothing, like a merino sweater and 100wt fleece pants, socks, fleece gloves and hat? Let's say again there is a tarp overhead.
I think once we get to #3 you might be able to get down to 40F and still get a comfortable nights sleep. Depends on some factors like still air, and whether we are talking about the general outside temperature or the temperature in the microclimate you have chosen for yourself under a tree on a hillside versus a rocky beach exposed to the night sky. It is easy to get mislead however, and as you get acclimatized to sleeping colder you might actually be closer to trouble. Some folks might think they crank out more heat while sleeping, and they might be right to some small degree but there are likely other factors at play. For example, wool can absorb moisture slowly, the moisture your body gives off even when you are sleeping and not sweating, and feel warmer that way. So it is about 10% warmer than its r-value alone, but only as it is gaining this moisture. It would have to be dried out before the next night, It can gain 35% of its weight without feeling wet, so the 20oz of wool might weigh 18oz when bone dry, and 24oz when it contains 35% moisture but still feels dry, not damp. Another factor is that your outer layer of skin can gradually drop in temperature from the time you go to bed and the time you wake up, and you can be acclimatized to tolerate lower skin temperatures and still sleep soundly, so even if you are only generating 80kcal per hour you can lose heat at a rate of perhaps 120kcal per hour over 6 hours before you wake up and need to warm yourself up. Of the 40kcal/hour deficit, 15kcal/hour might be made up capturing the moisture in your wool blanket, and 25kcal/hour might be made up by the heat lost from your skin, and it might be 6 or even 8 hours before you need to do something about that. The thing is this isn't hypothermia as your rectal temperature might still be fine, but you would be closer to hypothermia when you do wake up, as you have used up some of your natural built in safety blanket, like what allows you to nip out to pee in the middle of a frosty night.
I guess my point is that it is easy to get a false sense of security after some practice and a 6 hour test in the backyard, but there is a difference between being conditioned, and being safe in real conditions. Always good to find out how the human body works though. On the other extreme there are likely folks that think they are cold sleepers and need big sleeping bags, but they are probably actually sweating at night, more than they need to. They might have more safety margin though, but perhaps also more at risk in some ways. Apparently cold acclimatized folks have less blood volume in winter, so they are less prone to heart attack or stroke caused by high blood pressure and a sudden cold shock. Perhaps moderation is best, in all things, including moderation. <img src="/forums/images/graemlins/wink.gif" alt="" />
Here is that iceman runner. I wonder how he sleeps?
http://www.youtube.com/watch?v=madoDvtKEesp.s. Extrapolating to colder temperatures.
I think once you have figured out what you need for 40F, which might be half of what most people recommend at 40F, you would still need to add the same number of inches as everyone else for every 10 degree drop in temperatures below that, or about 1/2" total loft (top and bottom) for every 10degF. I think it would be dangerous to assume that if you need half the bag at 20F then you might only need half the bag of someone else at -20F. Everyone cranks out about the same kcal/hour per square meter. The only difference is that some people might be using the body as an extra 1/2" of insulation, and others might be sweating a little and negating 1/2" of their sleeping bag, but I think from that point on everyone needs about 1/2" total loft (top and bottom) per 10 degF.
Here is Mark Verber's page on sleeping systems which is really well done. I guess I am saying that acclimatized folks could get acclimatized to using just 2" of total loft at 40F, perhaps even just 1" total loft, but they would still need to add 1/2" total loft per 10deg the same as everyong else, and they should probably put some of that safety factor back into their bag and not plan on waking up cold on an extreme night on a long trip. I think its safe to skimp in summer, below the treeline, if you know its a short night and the temperature can't drop below 40F. You can always get up and dance. Winter is a different story. I think the Army recommendations below 40F are solid.
http://www.verber.com/mark/outdoors/gear/sleep-system.html
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