What does it mean if your entire body is itchy all over for hours on end?

This is head to toe itchiness on both hemispheres of my body and it has no visible cause such as bugs or irritants. There have been no changes is routine, such as stress, diet, laundry detergent, soap, etc. This only happens occasionally (about once every 3-4 weeks). I originally thought it was an allergy and would take Benedryl to no avail. When I get it, the itchiness prevents me from sleeping at all, since I will feel the urge to scratch about every 20 - 30 seconds. It seems too idiotic and vague to see a doctor about, so I'm wondering if anyone out there has dealt with something like this.