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January 2012 Issue
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A short history of… sleeping comfortably

Finding something soft and warm to sleep on is essential to a good night’s sleep in the outdoors. By Ross Millar

It’s almost innate, the knowledge to separate oneself from the hard surface, be it for easing a body bruised and battered from a heavy pack or to insulate oneself from the cold ground.

Te Rangi Hiroa in The Coming of the Maori on the chapter on Houses wrote “…fern was spread for bedding. The fern was covered with coarse flaxen mats, which were covered with finer mats when the house was occupied”. Similarly my tent floor last summer, on a trip up the East Sabine, was spread with a bed of old dry fern leaves and I slept soundly and relatively comfortably.

Laws of physics state that if there is a temperature differential then energy in the form of heat flows from a higher temperature to a lower temperature, the greater the difference the faster the energy flows. Lying still our bodies produce about 70-watts of heat output, comparable to a mid-range incandescent light bulb. If the air temperature is 10°C and our body temp is the average 37°C then the laws of thermodynamics will be trying to bring our body temperature down to 10°C. We attempt to stop this process by insulating ourselves with clothes during the day and a sleeping bag at night. Often the insulation provided by the sleeping bag and the ground is not enough to stop us losing heat energy by convection to the ground and we add some material layer to insulate us from the ground. Dry grass might be OK but it by the end of the night as been compacted down and probably isn’t providing much insulation nor softness.

An air mattress such as a Li-Lo gives some softness but the insulating properties aren’t that great since the air inside moves around as it looses heat to the ground. Closed cell foams and air mattresses with insulating material inside counter that air movement, have reduced thermal conductivity and thus provide better insulation.

In 1931 Stefan Mangold applied for the international patent on an ‘Air-tight Closed Hollow Body’ – the mattress that would become known by the brand Li-Lo, a joined tubular construction that superseded unbaffled inflatable pillows and mattresses made from all manner of materials, including animal skins, from as early as the 15th century.

Plastic polyethelene was developed by British chemists in 1933 and post war it was blended with copolymers and foamed with gases that enabled the cell components of the foam to become closed like lots of small balloons joined together. The foam gained the descriptive name ‘closed cell’ or ‘sealed cell’ foam and has the advantage of being water resistant, providing good thermal resistance, having strength and being inexpensive to produce. It wasn’t immediately of appeal to trampers and climbers though – and even in 1953 Sir Edmund Hillary and Sherpa Tenzing still carried air-mattresses to Camp Nine before summiting Everest.

In the early 1970s that Jim Lea and Neil Anderson of American company Cascade Designs, patented the Therm-a-Rest ‘self-inflating’ air-mattress. Self-inflating air-mattresses are made of open cell foam, a slab of which is contained within an envelope of airproof fabric with a valve in the corner. Their weakness is, of course, to the envelope being holed and the mattress deflating. On the other hand, even crampons will do little but cosmetic damage to closed cell foam slabs.

No mattress on its own will make you warm – it can only keep you warm in combination with other factors: your energy/heat expended, the quality of your sleeping bag and the environment you are in. They can, however, make a massive difference to comfort.

Value every minute of sleep you get in the outdoors as golden moments; some days you won’t manage to get too much and it can impact on the day ahead.