Bit late to the party, but here goes:
my 1-season sleeping bag turns out insufficient for the spring snow that surprised me at nightfall.
Well, that one season the sleeping bag refers to is not spring...
What should be done in such conditions?
In any case: Eat, preferrably hot food: you need your body to have sufficient fuel to produce heat to keep you warm. Sugar (sweets) before you start cooking give you energy that is avalable almost instantaneously, until the longer lasting nutrients from a full meal kick in. I've also found protein-rich stuff to really help in miserable conditions (e.g. a mug of thick, sweet cocoa with extra milk powder or protein powder can to miracles).
Exhaustion including being hungry are major contributing factors to hypothermia (and while we're at it, the conditions that got you exhausted & hungry often also favor dehydration).
What should have been done before: the day's tour should have been kept sufficiently easy/short so you are not exhausted. I.e. you have physical and mental reserves to take action during the night as required, once you're "refueled".
Break camp and hike out/to the next hut or at least in that direction.
Unless the conditions do not allow safe hiking out (e.g. ongoing snow storm, whiteout, you being night-blind) this is often the safest action you can take when you really do not have sufficient equipment for the conditions you find yourself in. Even in not light-polluted areas snow on the ground gives me sufficient light to hike. You want to hike slowly, so as to avoid exhaustion and accidents. Put on sufficient clothing to keep warm at a slow speed.
If you decide the situation is not that bad, so you can stay in camp:
Close your shelter as well as possible. With a tarp, go for a low setup with steep roof (because of snow), e.g. a bivy or forrester setup or a variation of the body bag with closed foot end or both ends closed.
The smaller the tarp space is, the warmer you'll be. If you close the head end, you'll have moisture, though (from breathing).
If "good insulation below" refers to twigs & leaves, you can add branches and leaves onto your tarp. (I assume surprise spring snow to be wet and thus not very suitable for insulation)
Put on all your dry clothes. I'd leave moist raingear outside as additional blanket.
If the sleeping bag is too small & tight for that, put them outside as blankets
After eating and before crawling into your sleeping bag, make sure your're warm - including toes, finger tips, ears and nose. If necessary, some excercise can help.
When waking up cold:
- if you need to go to the loo or are hungry or thirsty, take care of that, then do excercises until warm again.
- If there is no other need to leave your sleeping bag, static excercises are good for generating heat as well. Know a variety that warms you up.
There is no whatsoever point in waiting to get warm in the circumstances you describe, so take appropriate action.
Keep in mind that you need to act appropriately with mild hypothermia: it is the last (only) stage of hypothermia where you can rely on your brain functioning sufficiently well to take good decisions.
The backpack can help in two positions:
- as you describe as additional blanket/toe box, or
- to close your shelter.
If your sleeping bag is sufficiently large to let your curl up, do that - I'd keep my nose outside because of the moisture problem you describe.
When you curl up in your sleeping bag, the lower part is unoccupied. Turn that either below for additional insulation from the ground or up as additional blanket, whereever it helps most.
If you really have to spend the night completely inside the sleeping bag, as you describe it will get moist, and this impairs the insulation. This means that you must hike out the next day.
Space blankets have been mentioned. A bivy sack comes in the same category.
Enabling alarm every 30min or one hour in order to prevent white death
That is not necessary or even counterproductive since it may disturb the little sleep you are able to get whenever you are sufficiently warm.
If you are generally healthy, well fed and not under the influence of drugs (meaning not only alcohol/recreational drugs but also medication), there is no danger of getting into hypothermia and dying during your sleep. You'll wake up reliably when getting cold.
BTW, even quite exhausted muscles will produce substantial amounts of heat when they repair themselves during rest. The repair processes need protein and general excess energy (= well fed, otherwise the protein will be used as energy source).
What is sometimes popularly described as "falling asleep and freezing to death" is IMHO a misconception. People with severe hypothermia do not fall into anything like normal sleep, they fall into stupor, become unconscious and comatose. AFAIK, also terminal burrowing (aka hide-and-die) is not a conscious action like trying to get warm and cosy in order to sleep.
how do people in quality sleeping bags sleep in the winter with all that sub-zero air that they have to breath?
Speaking for myself, cold acclimatization does play a major role: my nose (ears and fingers) take a few cold days to remember that they need to heat more when it gets cold. If you always spend substantial time outdoors, your nose will be used to work with those temperatures. Apparently, exposing face, neck and breast to cold air stimulates thyroid cold acclimatization (that's more general, though, we're talking weeks, not a single night).
I've slept in dry -2X °C on top of my tarp (so not to get soaked by snow I accidentally melt) without problems breathing the air in February in Winnipeg - those -2X °C were after all much warmer than the -35 - -40 °C in January... I did use a scarf, creamed my cheeks and nose well with vaseline/petroleum jelly and used the sleeping bag's hood. (Covering my nose makes me miserable because of the freezing moisture). That occasion was a test of my sleeping bag I did in the garden before I went for a proper tour, btw.
The human nose has no general problem with staying warm in calm cold air - as long as sufficient energy is available to generate the required heat.
At temperatures substantially below 0 °C, the air is always very dry unless there is fog. This helps a lot with staying dry which is in turn important for staying warm. I find camping at +2 °C rain/sleet much more difficult than at -20°C.
A few more points to consider
A one-season sleeping bag can make a very nice (though bulky and heavy) "upgrade" to another sleeping bag if it is sufficiently large.
I typically bring gear that is good for about 10 °C colder than I expect (unless I'm only a few steps from house or car). I'll thus usually sleep with a large tarp e.g. in a C-setup, the tent door completely open and often also the sleeping bag used as a blanket with the zipper open. This gives a safety margin where I can put on more clothes, close the sleeping bag, close the tent or tarp. There's also nothing wrong with taking an extra blanket along (other than bulk/weight).
I find it much easier to bring some more gear than to be miserable during the night.
While @gerrit is right that being tired but warm is better than being tired and cold (since you anyways won't sleep when cold), being tired is a safety concern as well.
Proper winter camping (on snow) has the advantage that a pulkka/toboggan mean that volume and weight of the gear is not very problematic.
The more extreme the conditions, the more safety margin should be kept. Not only in terms of gear, but also in terms of food and fuel and in terms of keeping sufficient personal reserve (physical and mental).
Many sleeping bags IMHO are badly designed in a peculiar way: their back has much less insulation. The "reasoning" behind this is that this part is anyways compressed and won't isolate. However, that works only for those very orderly people who lie down like a mummy and stay in that position on their back without moving much for the whole night. If you sleep on your belly or on your side - including curling up egg-fashion because you're cold, you need good insulation at your back.
My warmest sleeping bag has this issue, and in consequence I've been known to sleeping quite twisted: my back in the sleeping bag's belly side, the cap turned around as far as it goes or folded as head rest and scarf + woolen cap instead of the sleeping bag's cap.
Test your personal comfort temperature range for your sleeping bag by spending calm dry nights outdoors close to your home (with at most a tarp put as a roof to keep rain/dew away). Forget the manufactuer's claims until you know how to translate them to your camping habits.