Irritable bowel syndrome, or IBS, symptoms often seem worse upon waking, at least to patients affected by the disorder. According to gastroenterologist Chung Owyang, chief of gastroenterology for the University of Michigan Health System, in the 2008 edition of "Harrison's Principles of Internal Medicine," evidence supports this notion: Few people with IBS experience pain that interferes with sleep and, although constipation does not seem to change at night, bedtime diarrhea occurs rarely.
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Eating represents a common trigger for IBS symptoms. Since people can't eat while they sleep and eating breakfast is one of the first things many people do when they wake up, it's one explanation for why IBS symptoms seem worse in the morning. Eating triggers IBS symptoms in two ways. First, the arrival of food in the stomach triggers the colon to contract, in order to make room for the meal that will soon arrive--a response known as the "gastrocolic reflex." For patients with IBS, the gastrocolic reflex often sets off waves of pain. Second, substances in foods such as fat, caffeine, alcohol and even carbohydrates—especially carbohydrates from milk, legumes, fruits and some vegetables—provoke IBS symptoms in people with mild deficiencies in the enzymes that digest them.
Many of the hormones implicated in irritable bowel syndrome exhibit diurnal variation—that is they peak in the morning and taper throughout the day, or the other way around. For example, melatonin, which surges at night when it's time to go to sleep, reaches its lowest point in the morning when it's time to wake up. Melatonin is mostly produced in the gastrointestinal tract and, not surprisingly, a 2009 study published in the "Journal of Physiology and Pharmacology" found significantly different levels between men and women with IBS and those without the disorder, especially in the morning.
According to Owyang, several lines of evidence link IBS to emotional disorders and life stress. While stress does not cause IBS, it can make it much worse and even interfere with the response to treatment. From a physiologic perspective, nerve endings in the brain connect in the nerve endings in the gut, providing a direct connection between the two. Circulating stress hormones such as norepinephrine and epinephrine provide an indirect link. Stress subsides with sleep but often surges when a person wakes and considers the day ahead.