I used to get very tired after a bath and I am almost sure that it was because of not enough blood going to my head. The heat from the hot water dilates the blood vessels in your body except for your head which stays out of the water. This causes less blood to flow to your head and can make you lightheaded and dizzy. The same thing happens if you stay in a sauna for too long which causes the blood to pool in your legs and away from your head. I now keep my head under water except for my nose and mouth while wearing swimming goggles and I don't feel tired at all after a twenty minutes bath anymore. This must be because I am now getting enough blood to my head. This is the only change I made. Before I had to lower the temperature of the water and now I take a bath with really hot water and don't feel tired at all. Plus I noticed that my ph is higher too! It must be because I am pulling the acids from my head too. And am only taking one half of the Armour thyroid I was taking before and still doing fine. Amazing. I think the reason you don't get enough blood to your head if you stay in a sauna too long is because you're sitting up where you're practically horizontal in the bath tub when you keep your head under water. It's gravity.