Peristalsis and Sleep: How both work in tandem with circadian rhythms

Though it may be common to get out of bed at night to use the restroom, very rarely is it due to a need to empty the bowel.

Our brains and bodies are built, in fact, to "avoid the void" at night as we sleep. 

What is Peristalsis?

The way we digest our food is primarily based on a pattern of rhythms known as peristalsis.

First we introduce food into the digestive system by way of the mouth. We chew and swallow it using enzymes from saliva to break down the food, which becomes known as a bolus.

The bolus passes through the esophagus and enters the stomach, where it encounters acids which further break down the bolus.

The process continues into the intestinal tract. In the small intestine, the bolus continues to be reduced by digestion. When it finally reaches the large intestine, the bolus is more or less a waste product that is prompted through the bowel until it’s finally excreted.

One process that is common to nearly every part of the digestive system is the series of muscle contractions and relaxations that occur throughout the process to move the bolus from beginning to end (and in a one-way direction).

This movement has a wave-like quality, utilizing bands of smooth muscle to push the bolus through the system.

Peristaltic waves work regularly throughout much of the system, but tend to slow down considerably in the large intestine, which is designed to undergo only 2 or 3 periods of bowel evacuation daily, and almost never at night.

How does digestion change during sleep?

As noted in previous discussions about food and sleep here at the blogcircadian rhythms influence digestive system activity. Our body clocks use cues based on time, light, and activity—including mealtime—to prepare the body for sleep. [However, the circadian rhythms that are uniquely operated by the digestive system don’t need light to function.]

Research shows that sleep and circadian rhythms work in tandem to regulate appetite, metabolism, and absorption of nutrients. Eating also guides certain circadian oscillators” which kick in during digestion to help manage the hunger drive and send messages to the brain when hunger has been satiated.

The speed of digestion at night naturally slows, and fewer relaxations and contractions of muscles occur during nonREM sleep (though during REM, they can speed up). This is all part of an intelligently designed plan by our circadian rhythms to allow us to sleep for several hours at a time, unimpeded by the need to empty our bowels.

It makes sense, then, that when circadian rhythms are disrupted, this leaves the digestive system open to problems with regulation and function. These problems can present in any number of ways, such as:

When the “feeding clock” becomes disrupted on a regular basis—for whatever reason—internal mealtime rhythms may shift to adjust to this new schedule. 

This is especially the case for people who work night shift or graveyard shift; their circadian systems eventually shift to a much later period for mealtime.

For others, a shift to later eating, followed soon after by bedtime, can lead to problems down the line with digestion. The digestive system needs time to process the meal, but cannot be as effective if sleep follows too soon after a meal due to these shifts. This complicates patterns of metabolism, lends to imbalances of important hormones related to both sleep and digestion—leptin, ghrelin, melatonin, cortisol—and can lead to obesity and metabolic disorders like Type II diabetes, as well as all the digestive problems listed above.

Sleep deprivation itself can increase the risk for overeating, glucose imbalances, and leptin resistance. Add late meals to the mix, and the problems to one’s metabolism are compounded.

Only when changes in light, core temperature, and hormones in the early morning signal for wakefulness does the colon take its cue to return to its regular speed to eliminate wastes.

Sleep, saliva, and swallowing: the problem with acid reflux

After we’ve gone to bed, our saliva production and pH both drop to zero and we swallow far less (during the day we may swallow 25 times per hour, but while we sleep, we swallow around 5 times per hour).

Our swallowing muscle is not "asleep"; in fact, it is one of the very few muscles that retains its tone during REM sleep, while the rest of the body’s skeletal muscles are in a state of temporary paralyzation. This is to prevent any problems with aspiration, or the "breathing in" of substances (water, acid reflux, excess saliva, etc.).

However, another muscle critical to the digestive process does lose tone during REM sleep. The lower esophageal sphincter (LES), a valve which works to prevent any reflux of acidic contents from the stomach from rising into the esophagus, becomes floppy during REM and is partly to blame for problems with GERD.

During sleep, the tissues of esophagus may become more vulnerable to the presence of these displaced gastric juices. With fewer swallows to help keep the contents of dinner down, and far less saliva to help neutralize the acid, this can become a real problem for late-night eaters.

Melatonin: it's not just for sleep

Melatonin, a hormone normally associated with sleep, also has a circadian-related influence on the digestive system. Its presence helps to improve the quality of the lining of the gut as well as synchronize the sleep-wake cycles based on food intake and the rhythms of the digestion system. Melatonin reduces stomach acid, increases digestive system blood flow, and improves the lining of the gut.

But for melatonin to be released, the environment needs to be dark, so late-night meals and activities can delay this important process. Less melatonin means digestive processes can be delayed or interfered with in a way that could lead to stomach issues.

How to avoid digestive problems at bedtime

If you have serious and persistent problems with cramping, gas, diarrhea, bloating, or other gastrointestinal distress during the night, you are best advised to consult with your doctor.

Otherwise, here are tips for avoiding digestive problems at bedtime:

How to help improve your overall digestive process

Good health habits during the day can lead to better sleep at night. Here are "3 Fs" you can focus on in your daily health routine to aid your digestive system even as you sleep:

Researchers are still examining the ways that sleep and digestion function in tandem as part of the overall circadian system. Understanding how to address circadian rhythm dysfunctions could prove useful for treating gastrointestinal problems which may be linked to sleep problems.


ChronoPhysiology and Therapy 
MedLine Plus
UCLA Sleep Disorders Center

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