That red light is delivered by the vagus nerve, which is a large nerve that comes from the brain and stimulates the contraction of the stomach. That vagus nerve is also the main cable controlling the parasympathetic system, which is the relaxation section of your nervous system.
The key stop-sign ingredient here: A peptide produced in your gastrointestinal tract called CCK. Technically, it stands for cholecystokinin, but for our purposes, let's think of it as the Crucial Craving-Killer because its main purpose is to tell your brain that your stomach feels fuller than Dolly Parton's corset.
Without having to go through the chemical pathways of your body (your bloodstream), CCK acts as a very direct message and indicator of your fullness. (Leptin, a hormone that signals fullness, is more of a long-term indicator of your satiety; CCK provides a very short-term, intense message.)
After food spends some time in your stomach, it will slowly leave that reservoir and go into the small intestine via the duodenum, the first part of your intestines that come right after the stomach. That's when CCK (a peptide produced in your gastrointestinal tract) puts up a digestive detour sign—in a very clear physical signal that makes you feel full. It causes the pylorus—the opening at the end of the stomach—to slam shut; that keeps food from moving into the small bowel and that's how your stomach gets full physically—and how you feel full mentally.
One interesting note: High saturated-fat diets lead to less CCK sensitivity, so you do not feel as full as you should after eating a steak.
Find out more about this book:YOU: On A Diet Revised Edition: The Owner's Manual for Waist Management