My girlfriend in Melbourne and my father in law have both started earthing at night. Both of them told me after a day or two that they were burning hot at night. My father in law even pulled out the plug thinking it would cool him down (like an electric blanket).
However, I feel the answer lies with the blood which has become thinner. As we age, our blood cells become sticky and clump together rather than circulating separately. Contact with the Earth appears to create warmth in the extremities and body as a result of better circulation and relaxation. There is a study where blood was taken from several people by a Cardiologist who was studying the effects of earthing on the heart. After only 40 minutes of earthing, the "ketchup" sticky blood in almost all subjects was changed to almost a "red wine" consistency. Photos were taken of the results which I now show.
Dr Sinatra went on to do a professional study: