If muscle tone or circulation are not good enough, then the blood can't travel uphill to the brain sufficiently to feed the brain. Without sufficient blood to the brain, virtually all of our functions will be weakened. The cerebellum, the back part of your brain, is where every physical organ is regenerated. You cannot breathe, you cannot hear, see or taste, you cannot think properly, nor move any part of your body without your back brain getting enough blood flow. This is, as well, the first part of the brain to be adversely affected by gravity.
Animals instinctively feed their brains the blood that is needed by how they sleep.
An animal is always in a prone position during sleep, and its head falls lower than the rest of its body. One of the conditions that we find the most responsive to the slant board is prolapsus of the interbal organs. Any of us who have been on a traditional western diet, with refined foods, for any extended period of time, will have a prolapsed bowel. The transverse colon, which crosses over the abdomen, will dip in the middle, this forcing the waste products in the colon to actually have to go upward, against gravity. This almost always proves too difficult, and a prolapsed colon then becomes a clogged colon.