Cold showers really work!

Seven million years of human evolution were dominated by two challenges: food scarcity and cold. “In the last 0.9 inches of our evolutionary mile,” they write, pointing to the fundamental lifestyle changes brought about by refrigeration and modern transportation, “we solved them both.” Other species don’t exhibit nearly as much obesity and chronic disease as we warm, overfed humans and our pets do. “Maybe our problem,” they continue, “is that winter never comes.”......

Another 2014 study found that, even after controlling for diet, lifestyle, and other factors, people who live in warmer parts of Spain are more likely to be obese than people who live in the cooler parts.

From The Benefits of Being Cold, The Atlantic, 2014

An Easy way to get in shape fast is to end every shower with cold water. It's difficult but not impossible. Start from the feet up, 1 minute is enough. You get in shape really fast. 

My oldest son, who is 16, sleeps with the bedroom window open even in the winter! Despite being a good eater he is really skinny. I guess exposure to cold makes a big impact.