circadian rhythms - What influences the timing of human bowel movements in the morning?

The bowel movements are influenced by a lot of factors. For example, when you eat a meal it induces a movement in your large intestines, to defecate and clear up space for new food.

Also, there is MMC, migrating motor complex, which is responsible for the bowel movements when you are fasting. It causes a flushing effect, which prevents bacteria to overproduce in intestines.

So, the daily bowel movements are mainly influenced by the timing and content of the food that you eat. But as I said there are many other factors. The gastrointestinal system has a very complex nervous system. Even psychological factors can effect the bowel movements greatly, for example extreme physical pain may induce the symphatetic system and cause constipation.

Also caffeine may affect it, like many drugs do.