Dr. Peter Whorwell, a gastroenterologist at the University Hospital of South Manchester, was interested how he could measure the effects of physiological and psychological stress on the body. Methods to induce stress in the laboratory (e.g. immersion of a hand in very cold water) are difficult for several reasons. These methods may produce different emotions in different people and the participants in these studies often adapt rapidly to these techniques.
Hypnosis doesn’t have these disadvantages. It is a safe way to induce specific  emotions and it can easily be reproduced.
That’s why Dr. Whorwell decided to induce emotions by hypnosis and then measure their physiological effect on the gastrointestinal tract, specifically the colon. The subjects in this study (Whorwell et al. 1992) were 18 patients suffering from irritable bowel syndrome. A catheter was introduced to measure colonic motility, the contractions of the intestine. Hypnosis was induced by an eye-fixation technique and subsequent emotional states (excitement, anger, and happiness) were induced by suggesting them directly by the hypnotherapist.
The results of this experiments were that emotions such as excitement and anger increased colonic motility (the intestine contracted more frequently), and happiness had the opposite effect, a reduction of the contraction rate. This study might explain some everyday experiences such as abdominal cramps or more frequent defection during especially stressful events in our life. The results may also help to understand why hypnosis is effective in the treatment of irritable bowel syndrome.

Figure 1: Distal colonic motility before and after hypnotic induction
Reference: Physiological effects of emotion: assessment via hypnosis
Whorwell, P.J. et al., The Lancet , Volume 340 , Issue 8811 , 69 – 72