So when you try to go to sleep, that itch that wasn't too bad during the day now becomes unbearable. The most important of these C-fiber neurotransmitters is substance P , which dilates blood vessels and indirectly activates mast cells . These release histamines, which cause redness and swelling of the skin. Some allergens affect mast cells directly, aggravating the itch. C fibers also convey pain signals, and for decades scientists considered itching a dull form of pain.
Dermatologists now regard itching and pain as different mechanisms.
Itching, however, elicits scratching. Opiates can both relieve pain and promote itching. And itching and pain can occur simultaneously at a single bodily site. Scratching, as Nash recognized, is taboo. While occasional, discreet scratching probably won't raise an eyebrow or a nose, repeated, public scratching can get one arrested.