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.
A person in pain tends to avoid touching the painful areas on his or her body. Itching, however, elicits scratching. Opiates can both relieve pain and promote itching. And itching and pain can occur simultaneously at a single site. Scratching, as Nash recognized, is taboo.