It's a system
Consider the clothes and the sleeping bag to comprise a "sleeping system"...
The functional demand on a sleeping system is to enable the user to
obtain a specified period of sleep in a given thermal environment. The
required insulation bears a linear relationship to the environmental
temperature ( ~ 4 clo/20 °C decrease in environmental temperature).
What's a "clo"?
The insulation of clothes are often measured in the unit "clo", where
- clo = 0 - corresponds to a naked person
- clo = 1 - corresponds to the insulating value of clothing needed to maintain a person in comfort sitting at rest in a room at 21 ℃ (70 ℉)
with air movement of 0.1 m/s and humidity less than 50% - typically a
person wearing a business suit.
(i.e., 0.155 m²K/W)
One clo unit of insulation can be defined as that insulation which not
only allows, but requires the transfer of 10 kcal/hr for an average
man (1.8 m²) for each °C difference between his mean skin temperature
(Ts, which can be defined for comfort at 32 °C) and ambient
temperature. Therefore, at an ambient air temperature of 0 °C the
non-evaporative heat loss will be 320 kcal/hr with a 1 clo sleeping
system; it will be 160 kcal/hr with a 2 clo sleeping system, etc.
How much heat?
Consider a soldier of average size; i.e., weight of 70 kg and height
of 173 cm, which corresponds to a total body surface area of 1.8 m².
Such an individual produces about 72 kcal/hr while sleeping (i.e., 0.8
MET) and loses about 25% of this amount from the body by respiration
and evaporation of the body water diffusing through the skin, which
typically maintains a minimum 6% wettedness; i.e., a skin relative
humidity of .06. This leaves 54 kcal/hr to be lost by the body by
non-evaporative heat loss through the sleeping system to the ambient
environment if body heat content is to remain unchanged.
What's the target measure?
Two definitions can be used for "adequacy" of a cold-weather sleeping
system: (1) "comfort", which implies that the sleeping soldier loses
just the 54 kcal/hr by non-evaporative avenues, i.e., can maintain
heat balance without shivering or sweating; or (2) "six hours of
restful sleep", which allows him to incur a total body heat debt of 80
kcal during a six-hour sleeping period and, therefore, to lose 67.3
kcal/hr, i.e., 54 + 80/6.
More insulation is more
[T]he highest insulation values obtained to date for a sleeping system, ca. 8 clo (for the U.S. Army's Extreme Cold LINCLOE, sleeping
system when used by a man sleeping in clean long underwear and socks
with the insulated air mattress), can be extended by adding
additional clothing items to achieve a 9 clo level. It was possible
to approach a 10 clo level of sleeping system insulation when the most
sensitive and heat loss prone areas of the body, i.e, the hands and
feet, were provided with supplementary warm sleeping gloves and
bootees. Conversely, much lower values of insulation were obtained
when just the long underwear and socks were worn and the extreme cold
LINCLOE sleeping bag was used on bare ground, without the insulated
air mattress which is part of the sleeping system; on a bare cement
floor, the insulation dropped from about 8 clo to less than 6 clo.