Whoa! it’s believe it or not time – this has to do with the jejunum.
Seems it may be the source of both SLEEP, and fear reflex VOMITING.
Book on sleep Principles and Practice of Sleep Medicine Chapter 27 (online)
These thoughts raise the issue of whether there may be changes in the GI system with food ingestion that could produce a hypnotic effect. Along these lines, an intriguing observation was made by Alverez in 1920. He noted that distention of a jejunal balloon caused his human subject to drop off to sleep. The hypnotic effects of afferent intestinal stimulation have also been documented in animal studies. Perhaps the most notable work was a study which induced cortical synchronization in cats by both mechanical and electrical stimulation of the small bowel.
73. Alverez WC: Physiologic studies on the motor activities of the stomach and bowel in man. Am J Physiol 1920; 88:658-660.
74. Kukorelli T, Juhasz G: Sleep induced by intestinal stimulation in cats. Physiol Behav 1976; 19:355-358.
This too from wikipedia: If the jejunum is impacted by blunt force the emesis reflex (vomiting) will be initiated.
Finally most of the melatonin comes from, not your head but your gastrointestinal tract. Melatonin is a hormone that induces sleep.
Levels of melatonin rise as the body temperature falls, to encourage feelings of sleepiness. The opposite occurs to wake us up.
“The concentration of melatonin in the gastrointestinal tissues surpasses blood levels by 10–100 times and there is at least 400× more melatonin in the gastrointestinal tract than in the pineal gland.”