Todd Becker pointed me to this post which is negative about the notion that cold showers raise mood (“empty science”) but you can ignore the negativity and go to the comment that gives a long list of studies that support the idea. Todd has blogged about his use of cold showers.
Todd calls this hormesis. About the mood-raising aspect of cold showers, I’m not so sure. There is a broad correlation between being sleepy and being in a bad mood. So anything that wakes us up is likely to improve our mood. But if cold showers improve one’s response to stress of all sorts — which is less clear — it does seem like hormesis in other contexts. When I think of hormesis I think of two sorts: intra-cellular (e.g., x-ray-like radiation breaks stuff, activating repair systems — radiation hormesis) and extra-cellular (microbes in fermented food activate the immune system). But there are other examples of similar stuff: exercise breaks muscle fibers (which is why you shouldn’t exercise the same muscles two days in a row) and longer-term increases them; bones when broken grow back stronger. If we need a certain amount of thermal or other stress to properly respond to stress that would be another example.