Why do we feel sleepy after we eat? There are two components to the so-called “food coma”: neurological and hormonal. When we eat, the food mashed up by your teeth is swallowed down the oesophagus and into the stomach, where it is churned in a vat of very strong hydrochloric acid. The acid dissolves the food into liquid form, which is then sent to the small intestines. Here, the chemical components of the food such as carbohydrates is broken into simpler blocks, such as glucose. This is then absorbed into the bloodstream.
The body can actually sense when you have swallowed food, as your stomach stretches and sends signals to the brain. This triggers the activation of the parasympathetic nervous system (opposite of the sympathetic nervous system, which is the “fight or flight” system). The parasympathetic nervous system is involved in digesting and rest. It stimulates stomach acid production, gut movement and even reduces your energy level so that digestion can happen smoothly. In old people, it can even decrease blood pressure enough to a point that they feel dizzy (much like head rush). This is the neurological component.
The hormonal component is linked with the absorption phase of digestion. To deal with the increasing level of glucose in your blood, the pancreas secretes insulin. Insulin rounds up the glucose in the blood and stores it away in cells to normalise the blood glucose level. In this process, it also stimulates the uptake of certain amino acids (building blocks of protein) into cells. However, it leaves out one type of amino acids called tryptophan. Because there is more tryptophan in the blood compared to the other amino acids, your brain decides to use this to build more proteins. Tryptophan is converted in the brain into a very important neurotransmitter called serotonin, which is then converted into melatonin. Melatonin is a neurotransmitter involved in triggering sleep. Therefore, through this extremely complicated pathway, food causes sleepiness.
At face value, this makes it look like increasing intake of tryptophan may help induce sleep. It is commonly said that turkey meat and bananas help you sleep because of tryptophan. But this is an urban myth as neither of these foods are particularly high in tryptophan and there is no evidence to suggest that tryptophan itself helps you sleep. Then again, melatonin supplements have some evidence supporting it as a sleep aide. This shows just how complicated the human body can be.