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A now-deleted answer to this question suggested using chemical heating pads in sleeping bags at night to provide added warmth. The product suggested is the 'Thermacare ... Fe-loaded material' I looked and it seems to use a chemical reaction to create heat over a period of about 8 hours.

ThermaCare® technology combines iron, oxygen, water and salt in single-use HeatWraps to produce real heat. Source

"Do not wear while sleeping" is one of the instructions on the official product usage label from the manufacturer. But, as long as it is just in the sleeping bag with me and not stuck to my skin it should be fine, right?

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It is true that the instructions say not to sleep with the Thermacare on you. Most electrical hot pads have the same warning. For some people, particularly those with compromised circulation, the heat directly on the skin can, indeed, result in a burn.

As a warming device in your sleeping bag, the product does not need to be directly on your skin. It could be over a layer or layers of clothes. It could be wrapped in a poly jacket. It could be shoved down at the bottom of your bag. It still pumps out heat inside the larger insulating envelope of your bag.

Now, before doing so, you probably should use one, on your skin, during the day to see how you, personally, respond to the output. I, for one, have no issues wearing one directly on my skin, and have done so many times when my back has been acting up. Once assured of daytime use, you can try it inside the bag, but off your skin. I am now confident of my personal response, and wear it right in my back.

Two things to consider further. One, I normally sleep on my sides, not my back, so I don’t have the product sandwiched with my body weight between me and my bag/pad. Second, I’m usually at 8000 feet or higher elevation, so the reaction rate may be lower given the reduced atmospheric pressure.

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    A couple things to think about adding to your answer: If a person is sleeping with their head inside the sleeping bag, what concerns are there for breathing in a confined space with the chemical reaction occuring? What happens if/when the products reaction chamber is broken inside the sleeping bag while the user is asleep? – James Jenkins Jan 2 '18 at 17:35

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