Sleep is one of the best parts of an expedition. You've toiled all day to an intensity which drains the life out of you in conditions which sap your energy. Because of this, getting a good night's sleep is one of the vital aspects of any journey. The noise of the wind, the extreme cold and moisture can make this very tricky indeed. Thorough research and preparation is therefore vital. The choice of what you sleep in is a good place to start.
Polar expeditons are not all made the same. Some are at altitude - some at sea level. Some at -40 degrees and some which vary from freezing to thawed temperatures. This makes the choice of a sleeping bag complicated and also critical to the success of a long expedition. Poor quality rest will degrade someone's ability to work in extreme conditions over many weeks or months. Here are some of the decisions that I make when choosing a sleeping system.
- Down bags -
Bags filled with goose or duck down are a traditional method of creating insulation through loft and a warmed air layer. It's enormously lightweight and allows the bag to be compressed down very small for storage in a sledge or rucksack (I tend to keep my tent-bag in a rucksack on my back - partly so I have a life-saving sleeping bag should my sledge be lost in a crevasse or lead). They do however perform less well in the wet. Moisture from a sleeping person and items drying in a bag will move through the inner lining and into the down (unless a VBL is used). This wet down will then lose its loft and stop insulating to anywhere near its original ability. This moisture will most likely freeze on the down and will not escape through the water-resistent outer shell easily. Water can also enter the bag through this outer shell which, by way of being breathable, is not fully waterproof.
Once water and ice-logged, drying out a down bag is difficult. On icecaps where there is potential for reasonably flat, stable surfaces and periods of sunshine, the bag can be left on top of a sledge and the ice allowed to sublime out. The down will then recover and loft up. On sea-ice or heavily crevassed ground, it's impractical to have the sleeping bag strapped to a sledge and so drying it out is harder.
- Synthetic bags -
There is an alternative; using bags which replace down filling with other man-made fibres, often Primaloft or similar. These are a lot heavier and harder to pack down than their down counterparts. It's harder to therefore get such a low temperature rating for a given weight. One great advantage of these bags is the fact they keep their insulating properties even when wet or ice-logged. This is great for when it's impossible to stop moisture entering from within (e.g. when having to dry lots of clothing inside the bag) or when the ambient temperatures are straddling zero degrees, meaning that ice/frost melts and enters from the outside.
VBLs (vapour barrier liners) are waterproof bags that sit inside a sleeping bag and stop moisture from a sleeping person entering the bag. This can result in a clammy night's sleep and a silk liner within can avoid the 'plastic bag' sensation. Socks and gloves that need drying and keeping warm are usually put between the VBL and sleeping bag.
So, the big question is what would you use for different conditions? There are various opinions on this and no clear answer with so many variables. Just because, for example, you're skiing over the sea, an Arctic Ocean expedition would not necessarily need a synthetic bag just to protect from water as the temperatures are averaging so low - there are other factors.
Greenland icecap (spring) -35 to -5 degrees - DOWN
e.g. Mountain Equipment Snowline, Iceline or Everest depending on how warm you sleep
Greenland icecap (summer) -20 to +25 degrees - EITHER (down if weight-conscious)
South Pole/Antarctic (summer) -40 to -10 degrees - DOWN
North Pole (spring) -55 to -10 degrees - SYNTHETIC (due to lack of opportunity to dry out bag during day)
Iceland/Norway (winter/spring) -25 to +15 degrees - SYNTHETIC (due to wetness of freeze-thaw alternation)
(NB Northern Outfitters, a USA-based manufacturer, are producing impressive equipment but I've yet to test them)
Essentially, synthetic is the 'safe' option but is heavier and harder to achieve low temp ratings with than with down bags. Hopefully the right decision results in a good night's sleep!