There seems to be a big push in the fitness industry to go back to our evolutionary roots. There's an interesting book out right now that takes an interesting approach to fitness: embracing extreme temperatures and eschewing modern comforts.
It's an interesting concept, and one that is picking up steam. With Tough Mudder and GoRuck competitions growing in popularity, there's somethign to the concept of pushing our boundaries of comfort to improve our physical and mental beings.
The author of What Doesn't Kill Us has himself walked the walk in order to investigate the book. He personally climbed Mt. Kilimanjaro in nothing but a pair of sneakers and running shorts. A quick check at Wikipedia tells me that the average temperature is about 25ºF during the day at the summit.
Anyway, it looks like an interesting book, and one that I'll be checking out.