If you don't sleep well, your body doesn't recover well from the day's exertions in time to perform all over again. The effect snowballs over 80 days on the ice until exhaustion takes over.
A key danger on extended cold weather expeditions, is your sleeping bag losing its insulation. Goose down is the material most commonly used for sleeping bags in the Antarctic, as it has the best weight/insulation ratio available.
However, it loses pretty much all its insulation (or to be more accurate, its ability to "loft" and trap dead air between you and the cold air outside the bag) when it gets wet. Some synthetic materials are as warm, and can retain their insulating abilities for longer when wet, but are heavier and less compressible.
As we'll be hauling around twice our bodyweight in sleds (known as pulks), weight is a massive issue and so down gets the nod.
The problem is that your body's natural processes can unwittingly turn your lovely warm and light down sleeping bag into a bed of ice over time. Our bodies give off insensible sweat when we sleep, even if we aren't aware of it. This can build up in your sleeping bag and will cause real problems if you don't do something about it, or if your bag has a so called "waterproof" shell on the outside. As likely as not, the claimed breathability of the protective shell will fail at extremely low temperatures, trapping the water in the down and turning your bag slowly but surely into an ice cube.
One way of countering the problem is by sleeping in a vapour barrier liner, which is a sealed, impermeable liner that stops any moisture entering the bag from your body. However, they tend to be clammy and in some cases as noisy as a crisp packet. We'll be bringing vapour barrier liners as a matter of caution and because they boost the temperature rating of the sleeping bag on really cold nights.
By the same token, the sleeping bag shell should be water resistant to a degree, as you don't want any water or soup spillage to ruin part of your bag's insulation. Materials such as Pertex Endurance are a great compromise. It breathes well, keeps the goose feathers from escaping and protects them from splashes, whilst being light and strong.
As for the design itself, we've all gone for Rab bags, due to their superb design, light weight, strength and no frills practicality. Will uses a Polar Expedition bag, which is incredibly warm but not the lightest, whilst Henry and I have gone for the Summit 900 bag (above), which is still dead toasty but a tad lighter. It all depends on how hot you sleep. Although the temperature outside will be viciously cold, especially with the windchill, inside the tent there will be a greenhouse effect due to the 24 hour sunlight of the Antarctic summer (as long as there are no clouds), which means that our sleeping bags need only be good for -25C. If we need more insulation on overcast nights, we can use extra clothing.
We'll each be sleeping on two Thermarest mats. The bottom mat is called a Ridgerest and is made entirely of foam. Next up is an inflatable mattress. The thicker the mattress, the warmer and more comfortable but the heavier to lag around. I favour a large women's Prolite 4 mattress, which is the same size as the men's regular but has extra insulation.
Thermarest Prolite 4 - in pink for the ladies
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