Luxury! Try cryogenic chamber therapy. Guess this doesn’t qualify under “lengthy periods” but it sounds pretty “interesting”.
[Quote=Wikipedia]
The chamber is cooled, typically with liquid nitrogen, usually to a temperature of −120 °C (−184 °F)—although temperatures of −140 °C (−220 °F) or even −160 °C (−256 °F) have been used.[3] The patient is protected from acute frostbite with socks, gloves and mouth and ear protection, but in addition to that, wears nothing but a bathing suit. The patient spends a few minutes in the chamber. During treatment the average skin temperature drops to 12 °C (54 °F), while the coldest skin temperature can be 5 °C (41 °F). The core body temperature remains unchanged during the treatment, however it may drop slightly afterwards. Therapy triggers the release of endorphins which induce analgesia (immediate pain relief).
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