Cassandra, the beloved protagonist of the classic 1948 coming-of-age novel, "I Capture the Castle" writes: "Noble deeds and hot baths are the best cures for depression." While most doctors wouldn't go that far, an hour-long bath has benefits that go way beyond the relaxation and stress reduction most of us are familiar with.
According to a recent study from the National Centre for Sport and Exercise Medicine at Loughborough University in the U.K., “An hour-long hot bath can boost metabolic health and cause an anti-inflammatory response similar to exercise.”
It was a small study with surprising results: Ten sedentary men, attached to glucose monitors (which recorded changes in their blood sugar for 24 hours) bathed in 104-degree water one day. The next, they cycled hard enough to bring their body temperature up by about a degree, which is what happens in a hot bath, too.
The bathers burned an extra 126 calories per hour (equivalent to a 25-30 minute walk). That was still less than the energy used for cycling the same amount of time, but was an 80 percent increase in energy expended over not bathing.
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http://www.mnn.com/health/fitness-well-being/blogs/how-about-bath-instead-workoutUsual caveats - small study, your mileage may vary, not valid where prohibited.