JR, Actually it does. When you consider that the human body is connected, it becomes a single “system” maintaining your overall temperature. Blood, and other fluids in your body circulate transferring available heat through out your body. It also carries any cooled fluids back into your body. Your head and brain MUST be maintained within a few degrees for normal functioning. In extreme cold, you can freeze fingers & toes, arms and legs before your body will let your brain get cold (go hyporthermic.) It is the only place where you have effective natural heat insulation, your hair, though there are others (armpits/groin) for other purposes.The current ISO EN 23537 (superseding the older EN 13537) recognizes this and requires a head covering to rate bags and quilts. Most manufacturers of sleeping bags also recognize this by including a hood on most mummy style bags. A few recognize that the user would actually have a seperate piece of equipment (beanie, hood, balaclava) in the interest of flexibility of use. 2-4 degrees C is a reasonable estimate.
As with all standardized testing, these are for an “average” person statistically, though the vast majority of people will never hit this theoretical “average”. Most will sleep colder or warmer than “average.” The new testing protocol recognizes this, too, giving a range of temperatures over a some group of tests.
