"Technically", Scientifically, 321a_140 and Zach140 are correct. Good thought on the Newton thing which is correct concerning weight, but do not forget the basic properties of gravity. The force of gravity is affected by two things, mass and distance. What this means is that more massive objects exert more force (pull) on each other. What it also means is that the closer objects are to each other, the more force (pull) they exert on each other. That is why you accelerate when you fall from a plane. Remember, Force=MassXAcceleration, not Force=MassXSpeed. What this means is that the closer you or any object are to earth, the more the earth pulls on you and the effect of distance is exponentially multiplied. What this means is that if you halve the distance, you do not double the force but quadruple it. What does all this mean? What this means is that when you stand on your head causing body material to move towards your head due to gravity and thus farther from earth when you are upright again if done quickly, the body material will be "pulled" less. Consequently, you could and should weigh slightly less. The question then becomes what is slightly, and is it enough to register on commonly used scales. Sometimes it works and sometimes it does not. I have seen it work either because of physics or luck, but I have seen it work and I have seen it work on digitals as well. That is the science which is indisputable and irrefutable (unless one is in denial).