Switching between cold and hot water in a shower can improve your circulation. When the shower is cold, all your blood goes to the organs to keep them heated. When the shower is hot, the blood goes to the skin to keep from overheating your organs. It's a way to get your blood flowing. And you'll be using half as much hot water.
NOTE: If you suffer from ill health, especially heart problems or are pregnant, consult a doctor before attempting this. The sudden burst of cold water can cause quite a shock.