According to the Center for Disease Control (CDC), "extreme cold" is any temperature significantly lower than your body is used to... in essence, they feel this is relative. On one hand I agree with them, since someone living in a subtropical zone is certainly not physically adapted to temperatures near or below freezing. On the other hand, extreme cold, I mean really extreme cold, has some hard delineations (in my mind at least) and those center around the duration, maximum exposure times, and the difficulty to maintain adequate heat.
A healthy person stuck in just barely freezing temperatures around 30F, no matter how unused to it they are, can easily survive those temperatures with a normal 2000 calorie diet and some warm clothing. However, a healthy person stuck at temperatures much below 0F couldn't survive on a 2000 calorie diet no matter how many clothes they had... because 2000 calories is simply not enough fuel to keep the metabolic furnace burning hot enough to keep the core body temperature above 95F (hypothermia). If your body can't produce enough heat from the inside, all the insulating clothes, blankets and gear in the world are not going to keep you warm enough and certainly aren't going to make more heat. You need fire or chemical heat source at that point.
Also, while it may be more difficult for someone in milder cold to continue working at those temperatures, the warmth produced from their exertion and the layer or two of warm clothes is more than adequate to keep them warm with just a little more food to keep the furnace stoked. In that temperature range, under those conditions, they could pretty much continue working at the same exertion level and duration. However, when you're working in sub-zero temperatures, you start running into the situation where your body forces you to decide whether to expend your energy on work or keeping warm... you simply do not have enough metabolic energy to do both for any extended period of time.
Even consuming more calories does not automatically tip the scales in your favor in the extreme cold. Chewing, swallowing and digestion itself requires energy, so eating more is subject to the law of diminishing returns. Eventually, you don't really gain any more usuable energy for your efforts just by consuming more and your body will start shutting down non-vital systems in order to conserve more... including, unfortunately, digestion.
Additionally, in milder cold, you have a much longer exposure time to the elements before your body starts to suffer permanent damage. Flushing (dilation of the blood vessels in the skin) and shivering (contraction of the muscles) is usually enough to circulate and generate enough heat to keep your surface tissues and extremities from freezing even if they are exposed, so long as you have some sort of warm clothing layer. You would have to be subjected to at least a couple hours of direct exposure, and several hours of indirect exposure (i.e. continued loss of body heat in covered areas like your extremities) before any significant or permanent damage was done. Manual dexterity and mental processes are not significantly impaired for several hours.
However, at sub-zero temperatures, any exposed flesh can freeze in minutes (even seconds) despite flushing and shivering. The body heat is simply getting wicked away too fast, and the body shuts down blood flow to those areas almost immediately in attempts to keep the core temperature above hypothermia -- this is what is known as "Frost Nip". Once deprived of blood flow and oxygen, your cells literally start to freeze and form ice crystals -- this what is known as "Frost Bite". At -30F, exposed skin can go from healthy, to frost nip, to frost bite in less than 5 minutes. At those temperatures, unless you can keep your core temperature high enough through a combination of food and clothing, you risk frost bite in even your covered extremities in less than one hour. Sustained exposure to these temperatures and lower, even covered and maintaining your core temperature, increases the likelihood of developing frost bite in your extremities 10% for every 30 minutes you remain exposed. Manual dexterity is affected almost immediately, and mental functions become impaired after only an hour.
While it may suck royally to be stuck at 18F in your normally balmy southern province, it's just not the same as being stuck at -40F. And routinely experiencing sub-freezing and sub-zero temps for a couple of few-day stretches in a more northerly province, is still not the same as being stuck continually below freezing for 3-4 months, and below zero for weeks if not months. For comparison, most "cold" regions in the Lower 48 experience sub-zero temperatures about as frequently as we at higher latitudes experience -50 or below. -10F vs -50F... that's a 40 degree difference, which more than the temp difference between most people's refrigerator and freezer compartments. Think about it.
I'm not knocking folks feeling the cold at temps I would consider warm. Not at all. In fact, I'm asking them to recognize how difficult their life is at those temps before assuming that I can do what they can do when I'm facing temps at least 4 times lower than they are. Just because they can go out and still manage to walk the dog for an hour, or chop firewood all afternoon, or even get their cars & chainsaws running at, say, 18F... does not mean that we can do the same things at -18F. That's a 36 degree difference, and it counts. At the temps we have to deal with, some fuels won't even ignite!! So, you'll have to excuse that we sleep 12+ hours while our bodies conserve energy, and can only work (or play) outside for an hour or less before we have to come inside to sit by the fire, eat some truly fattening snacks, and take a nap.
Even given the extreme cold limitations, I wouldn't trade my Alaska wilderness for anything! I chose it, and I'm not complaining. Just fighting a little judgmentalism that's been cropping up lately :D