We all know that we shrink a few inches over the course of a day, and that this is corrected when we sleep. Also I'm not sure but I think gravity plays a not insignificant role in causing sagginess to certain body parts over the years.

Therefore, hanging upside down would be a method of actively opposing these negative forces. I think Patrick Bateman in American Psycho used a rack fitted to a door frame in one scene to hang upside down, whist explaining the many benefits of doing this. More blood to the brain might even improve intelligence. I know there's been times when I've really felt like hanging upside down for a few minutes but there's a surprising lack of opportunities for this.