What is the definition of sitting? What about laying on a bed? Is that something completely different? Are they qualifying anything that isn't standing as sitti

What is the definition of sitting? What about laying on a bed? Is that something completely different? Are they qualifying anything that isn't standing as sitting? I'm an avid cyclist, does the "sitting" position have an effect on my death coming nearer? I think the more interesting question is what is happening physically while "sitting" that causes this?