How does the body’s digestive system normally work?
Like every other part of the body, the digestive system needs energy to function. This energy can only be obtained by consuming food, which is why what we choose to put into our bodies matters so much. 
When we eat, that food is processed and broken down by the digestive system until it gets to its final product… glucose. This glucose goes on to provide our body with the calories (energy) that it needs to go about all of our daily functions of life; not only moving around, but also breathing and having your heartbeat in a normal rhythm. 
In addition to processing food, the digestive system is also responsible for triggering other responses from our body as well. 
For instance, there are hormones released in response to the digestive system that make us feel full (also known as satiety), specifically, the hormone cholecystokinin (CCK, located in the central nervous system), amylin (located in the pancreas), and glucagon (also located in the pancreas). 
The digestive system also plays a role in why the body's blood sugar rises after eating or drinking, as well as the production of the insulin needed to move glucose from the bloodstream into the cells where it is used for energy.
Is this normal?
The first question people ask when they notice that they are feeling more tired than usual after eating a meal is, is this normal? The short answer to that question is that, while "food coma" is a common feeling, it is not actually “normal” in the sense that it’s not the response your body should be experiencing.  Some tiredness, especially after big meals, is definitely considered normal. As our bodies work to digest food, they reroute blood from other body systems into the digestive system, which is part of what causes that overall feeling of sleepiness. It may also trigger the release of specific hormones, like serotonin, that can cause post-meal sleepiness. In some cases, the foods we eat may also trigger the production of melatonin, which is a hormone well known for inducing sleep. While feeling a little tired after a meal is normal, feeling excessively tired may be a symptom of a larger issue that should be evaluated by a medical professional. Digestive health should always be taken seriously. 
Your diet may be to blame
In a lot of cases, feeling excessively tired after eating a meal is tied directly to the foods you are eating. All foods are digested through the same process, but that doesn’t mean that they all affect the body in the same way. Foods that are high in an essential amino acid called tryptophan (you may be familiar with this amino acid because of its inclusion in turkey and the “Thanksgiving meal” effect) may be responsible for that sluggish feeling after eating. Tryptophan is used by the body to help create serotonin, which is a neurotransmitter that, among other things, helps to induce sleep. It isn’t actually the tryptophan that makes people feel tired after Thanksgiving dinner, it’s the increased production of serotonin that can happen as a result of it. It isn’t always turkey that is responsible for this serotonin overload. Other foods, specifically those high in protein, can also cause post meal drowsiness. This includes spinach, eggs, tofu, fish, soy, and cheese, but they must be consumed in fairly high quantities to cause this reaction.  It pays to take a good, close look at your normal diet to see if you may be simply consuming too many foods known to be high in tryptophan.  Using supplementation to help with any potential vitamin and mineral deficiencies due to food intolerances or food allergies may also be helpful in certain situations. A focus on eating whole foods, like fruits and vegetables, fatty fish, healthy fats (like nuts and olive oil), and complex, high-fiber carbohydrates is great not only for gut health – but also potentially good for insulin levels and brain health.
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