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There is a popular belief that a coffee and a cigarette in the morning contributes to having a bowel movement at that time.
There is already somewhat of an answer for the question to why coffee seems to have laxative effects, however it's not very detailed and does not say anything about other factors like cigarette smoking.
Cigarettes also seem to contribute strongly to this effect (at least in popular belief). Is this a real, biological effect or just a placebo?
When thinking about the nervous system, most people think about the brain and spinal cord: the CNS. However, the enteric nervous system controlling gut function has another ~500 million neurons.
Among other things, these neurons coordinate the peristaltic contractions of the gut which act to move food through the digestive system.
Peristalsis is under the control of several neurotransmitters and neuromodulators. One of these is acetylcholine. One of the two major classes of acetylcholine receptors are known as the nicotinic acetylcholine receptors (nAChR). These receptors are so-named because nicotine, found in cigarettes, is a strong agonist for them (although their typical endogenous agonist is acetylcholine).
There is good evidence that nAChRs are involved in gut peristalsis. Antagonists of nAChRs reduce peristalsis Kadowaki et al. 1996 and agonists increase peristalsis Blank et al. 1999.
Therefore, you can expect that cigarettes, which contain nicotine, would increase gut motility and therefore make bowel movements more likely or more imminent. Of course, there can also be daily cycles of bowel function, so you cannot easily link smoking in the morning to a specific subsequent bowel movement.
References
Blank, E. L., Greenwood, B., & Dodds, W. J. (1989). Cholinergic control of smooth muscle peristalsis in the cat esophagus. American Journal of Physiology-Gastrointestinal and Liver Physiology, 257(4), G517-G523.
Kadowaki, M., Wade, P. R., & Gershon, M. D. (1996). Participation of 5-HT3, 5-HT4, and nicotinic receptors in the peristaltic reflex of guinea pig distal colon. American Journal of Physiology-Gastrointestinal and Liver Physiology, 271(5), G849-G857.
FOOD
A simple way to stimulate the bowel movement in the morning is to eat or drink something - this is known as the gastrocolic reflex.
The gastrocolic reflex is the physiological reflex that controls the motility, or peristalsis, of the gastrointestinal tract. It involves an increase in motility of the colon in response to stretch in the stomach and the byproducts of digestion in the small intestine. Thus, this reflex is responsible for the urge to defecate following a meal.
COFFEE and NICOTINE
Coffee can stimulate the bowel movement but so can water. Nicotine does not - according to this study:
Rectal tone increased by 45% 30 min after coffee intake and by 30% after water intake, but the effects of coffee and water were not significantly different. Rectal tone did not change significantly after administration of nicotine (7%) or placebo (10%).
