This has mainly to do with the lower metabolic rate and lower muscle ratio of women.

The body heats itself through biochemical activities called metabolism.
Non active muscles add ca. 20% to body temperature. If you are getting active with muscles work e.g. sport than muscles work produces heat up to 2KW. That`s the reason why you start sweating when you do a hard workout.

Around 15% of body heat is produced by the brain. Why the brain is hidden in the bone box called skull and isolation tissue like fat oder muscle tissue is missing you will start automatically wearing a cap when you feel loosing heat.

If the body temperature gets too warm, heat must be evaporated to secure the metabolism function. 90% of produced body heat will evaporate over the skin because it is the organ with the biggest surface(~2sqm). 10% of heat will be evaporated through breathing. Women who have generally a lower metabolism rate evaporate less heat over the skin which is the reason why they faster get cold compared to men. If they evaporate less heat over the skin, they need a warmer sleeping bag.

Women normally also have a lower muscles ratio than men. In relation to body weight women have a muscles mass of 25 -35%. Men do have 40 - 50%.

When you sleep your muscles aren`t active but the metabolism is still supplying energy in form of oxygen, glucose, fat and protein to them to keep them ready and in shape for suddenly actions (leftover from our evolution and the reason why we loose body weight if we are undersaturated with calories and why we gain fat/muscles if we are oversaturated with calories during sleeping ).

The difference of the muscle/body weight ratio and the lower metabolism is the reason why women feel faster cold. Think every men knows this in form of discussions to heat up the room or by cold feet in bed...

And it`s the reason why women`s temperature feeling is taken to norm sleeping bags.

Fair deal I think.

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