The digestive process is divided into stages.

The first stage, the cephalic phase, occurs in the mouth. When we see food, think about food, or are accustomed to eating at a certain time, our mouth begins to produce saliva that is rich in enzymes, making it easier for the stomach to do its job. The mouth produces even more saliva as soon as we start chewing, while the brain instructs the stomach to release digestive acids. Nearly one-third of the acid needed for digestion is released in this stage. Saliva secretion is circadian. It is most productive during the day, up to 10 times greater than it is when we sleep. The nighttime drop in saliva production helps us stay asleep, although it is another reason we wake up with dry mouth. Daytime saliva secretion neutralizes stomach acid that may come up through our esophagus into our mouth, but reduced saliva at night is not sufficient to carry out this task. Eating late at night can produce excess stomach acid, and if that acid comes back up the esophagus and into the mouth, there is not enough saliva to neutralize it. As a result, late-night eating can trigger acid reflux, causing inflammation of the esophagus and permanent damage to the esophagus, stomach, and teeth if left unchecked. 

Once food is properly chewed and swallowed, it travels down the esophagus and passes into the stomach, beginning the gastric phase of digestion. The acidic environment of the stomach is like a brewing vat, further breaking down food into microscopic particles. The acid is contained in the stomach by the sphincter muscle that is at the junction between the esophagus and the stomach. This acid is so strong that it can even kill bacteria found in raw food like salad. Excess acid production, even at the right time of the day, causes acid reflux. Diminished acid production is also bad, because it promotes the growth of dangerous bacteria that cause diarrhea. It also allows for incompletely digested food particles, which can trigger inflammation by the immune cells present in the gut lining. This is referred to as a leaky gut. Stomach acid production is typically high during the hours before bedtime, roughly 8:00 to 10:00 p.m. If morning stomach acid is produced at an arbitrary unit of 1, at night it reaches 5. However, when food is consumed during the day, your stomach acid production may go up to 50; eating the same amount at night may increase production up to 100. This means if we eat a modest meal in the evening, the stomach will produce a larger amount of acid than if the food was consumed at noon. Our food sits in the stomach for 2 to 5 hours, depending on how much we eat. 

Then it passes from the stomach to the intestines, where further enzymatic and chemical digestion continues. This marks the beginning of the intestinal phase. The intestines are not designed to handle the high acidity that is present in the stomach, so once the food enters the intestines, acid secretion is reduced and neutralized. Once food enters the intestines, it does not move by itself. Rather, it is squeezed along the digestive tract by muscles that surround the tube. This is called gut motility or gut contractility. An electrical signal from the gut’s nerve cells triggers the muscles to expand and contract. This produces a wavelike motion that pushes food through the tube. Once food is fully digested and the nutrients absorbed, the waste by-product reaches the colon, the last part of the gut, and exits the body as stool, a full 24 to 48 hours later.