When you sit on a hard surface, you can compress the nerves in the buttocks (sciatic) and create paresthesias...odd feelings. The ischial tuberosity bone onthe bottom of the pelvis can make the feet go numb because you are squeezing tissues between two hard surfaces.

People vary in their responses to pressure. These are called pressure palsies when severe. You might see that term on this board now and then.

When you retain fluid (due to too much salt--or hypothyroidism or pregnancy for women etc), your hands and feet may compress nerves in the wrists and ankles giving these sensations too. That is when I know I need my diuretic, which I don't like to take. One Lasix and I am good for another week or two. I just sprang for some nice pickles, so I am suffering today. This is much more common in women with their hormone fluctuations. So if you eat alot of salty foods, this can aggravate numbness/tingling if you already have that.