What Causes Feel Of Numbness At Backside Of Body?

Whenever I scratch a part on the backside of my body (back, neck, shoulder, etc) it feels numbish. Not completely numb, but it feels like it's "thicker" I work for 8 hours a day (standing and moving around occasionally) and when I am home I am usually laying on my back in my bed browsing the internet with my laptop on my lap.