But the relationship between sleep and digestion is far more intricate, and going to bed sleepy after a big meal does not mean you are going to sleep well. Here's some basic biology about digestion as it relates to sleep.
Your digestive system tends to also "go to sleep" when your body and brain begin to send out sleep signals. This means the muscular work that the bowel does slows down, you generate less saliva and swallow less, and stomach acid remains in the system longer, peaking in quantity at around midnight. This is due to a "digestive" clock in the brainstem which syncs with other clock signals in the brain related to sleep stages.
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However, if you have a very large meal late in the evening, just as your body and brain are closing down shop for the night, the digestive system gets confused or doesn't function properly.
With less swallowing and less saliva comes less opportunity for the body to clear away the stomach acid after it has done its job. Acid secretions that remain in the stomach after digestion can potentially travel out of the stomach, back up into the esophagus while you sleep, leading to the common problem of gastroesophageal reflux disease or GERD, which is marked by heartburn and indigestion. You wake up with a nasty acid taste in your mouth and a burning sensation in your throat and it can make it very hard to return to sleep. If you've ever had heartburn, you may have also noticed how the salivary glands can suddenly produce a lot of saliva that you swallow; this is the body's way to move the process along and clear out that acid.
But you aren't consciously thinking about swallowing while you are asleep, right?
When your body goes into REM sleep, all muscles below the chin paralyze, making it impossible to swallow against a rising flow of reflux (stomach acid that is still digesting your meal). What's more, you run the risk of actually inhaling some of this flow if it rises high enough in the throat to access your airway.
So what happens to your sleep quality on a night when you've eaten a large meal late? Either your body wakes from much-needed REM sleep to participate in some emergency swallowing, or you sleep through acid erosion in your throat, unaware that over time, this can create a lot of damage to the mucous membranes and the muscles that are meant to keep the stomach acid from reentering the esophagus, meaning the damage can be permanent.
It gets even worse for people who have asthma. They may actually inhale this acidic matter into their lungs, which is dangerous to their already compromised pulmonary system.
And what about the bowel? It's suddenly full of matter that needs to be moved out of the body precisely when the body's muscular system isn't primed to do the work. The unprocessed food waste waits there, as if in a traffic jam, and this can lead to trapped gas, constipation and pain.
There may be a link between Irritable Bowel Syndrome and late-night eating habits because the usual mechanisms for digestion simply aren't "awake" to do their job in regulating waste. People who awaken in the early morning hours with gastrointestinal pain may also be more likely to develop ulcers.
Going back to that original blood sugar-insulin imbalance described earlier, it's important to note that the body generally is meant to "fast" overnight rather than spend its time digesting things. (This is where the term "breakfast" comes from... in the morning, you "break" your "fast.") If you eat a large meal, then go to bed, this interrupts the normal fasting process and leads to dysregulation of glucose, insulin and other appetite-related hormones. If one falls into the habit of eating large meals late most of the time, they may find themselves battling with any number of metabolic disorders, including obesity and diabetes, as the body no longer can effectively process "energy" (calories) the way it is meant to process them.
'Tis the season to celebrate, often late into the evening and inside a food-laden landscape. Going to a feast after midnight mass on Christmas Eve won't automatically lead to all of these problems, so don't worry so much about a rare late meal. But if you find yourself turning to over-the-counter digestive aids like Tums or Gas-X or Pepto Bismol frequently during the holidays, you may want to reconsider your eating habits and how they might interact with your sleep, because if they have a negative impact on your sleeping life, they can eventually lead to systemic health problems down the line.
"Gastrointestinal Physiology in Relation to Sleep." Orr, W. and "Gastrointestinal Disorders." Orr, W. from Principles and Practice of Sleep Medicine, 5th Edition, eds. Kryger, Roth and Dement.