Why Do We Get Sleepy After a Big Meal?
β How the Body Switches to "Digestion Mode"
Have you ever been hit by a wave of sleepiness in an afternoon class or at work, after a hearty lunch? You weren't sleepy at all before eating. But once you're full, your eyelids start to droop. What is going on inside your body?
Compare a light breakfast with a big, filling set lunch. Did you notice that the sleepiness afterwards was different? Many people feel especially drowsy after eating a lot of carbohydrates, such as rice, bread or noodles.
This "post-meal sleepiness" isn't just your imagination, and it isn't laziness. It is thought to be the result of several changes in the body working together.
When blood sugar rises after a meal, the nerve cells that keep the brain awake are thought to become less active.
After a meal, the body is said to shift into a mode where the nerves in charge of rest and digestion (the parasympathetic nerves) are dominant.
Let's look at how these two changes in the body cause sleepiness after eating, one at a time.
Changing Blood Sugar Weakens the Brain's "Wake-Up Switch"
When you eat, the sugar from digested food is absorbed into your blood, and your blood sugar level (the concentration of sugar in the blood) rises. The brain is thought to contain special nerve cells that keep you awake. These cells are believed to become less active when blood sugar goes up. In other words, the rise in blood sugar after a meal indirectly weakens the brain's "stay awake" switch.
The Parasympathetic Nerves Take Over, and the Body Enters "Rest and Digest Mode"
Our bodies are said to have two kinds of autonomic nerves: the "sympathetic" nerves, which work when we are active, and the "parasympathetic" nerves, which work when we are relaxed and at ease. After you eat, the parasympathetic nerves are thought to take over to help digestion. When they are dominant, the body relaxes, and this is also a state in which sleepiness comes easily.
It is a sign that your body is deliberately switching out of its active mode to concentrate on the big job of digestion.
Digestion Itself Uses Energy
The work of digesting and absorbing food uses energy (calories) in itself. This energy spent by eating is called "diet-induced thermogenesis", and a certain share of the calories you eat is thought to be used up in digestion. Because energy goes to digestion, the body may have a little less to spare for other things (such as staying alert) for a while. This could be one contributing factor.
Eating a lot of foods that raise blood sugar sharply, such as white rice, bread or sweets, is said to make sleepiness stronger afterwards. On the other hand, eating in ways that curb a sharp rise in blood sugar, such as eating vegetables and protein first, or chewing well and eating slowly, may change how sleepy you get.
What You Can Check Yourself
- For a few days, note what you have for lunch (mostly carbohydrates, or plenty of vegetables and protein too)
- About 30 minutes to an hour after eating, rate how sleepy you are, for example on a 5-point scale, and write it down
- Compare your notes on meals and sleepiness
- See whether there is a pattern of meals that tends to make you sleepy
The link between what you eat and how sleepy you get is said to differ from person to person, so your own records are your best clue.
Summary
One reason we get sleepy when full is that the rise in blood sugar after a meal weakens the nerve cells that keep the brain awake. The other is that the parasympathetic nerves that aid digestion take over, and the body relaxes into "digestion mode". The two are thought to work together. The energy used by digestion itself may also play a part.
Sleepiness after a meal is not a sign of weak willpower. It is a sign that your body is working hard on the important job of digestion.
Incidentally, some animals don't digest food using their own body alone. How a cow can grow so large on grass is explained in Cows Only Eat Grass, So How Do They Get So Big?
For Those Who Want to Know More β Terms, Numbers and Links to TextbooksFrom middle-school science to active research, each topic is labelled by level
- Middle schoolCovered in middle-school science
- High schoolCovered in high-school basic biology
- High school+High-school biology, or advanced or sidebar material in textbooks
- UniversityUniversity-level specialist material (neuroendocrinology), not taught in high school
- ResearchTopics researchers are still investigating, not taught even at university as settled fact
Middle schoolTerms: Words Around Post-Meal Sleepiness
- Blood sugar level: The concentration of sugar (glucose) in the blood.
- Autonomic nervous system: The nerves that automatically adjust the work of the internal organs, without your conscious control.
- Parasympathetic nerves: Autonomic nerves that relax the body and promote digestion and similar functions.
High schoolChecking with a Formula: A Guide to the Energy Used in Digestion
We use a typical figure for diet-induced thermogenesis (the energy the body uses because of a meal) to calculate how much energy goes into digestion.
| In symbols | E οΌ Q Γ F |
| In words | Energy used for digestion οΌ energy in the meal Γ share lost to diet-induced thermogenesis |
| Where it comes from | It comes from the idea of an energy balance: the energy you eat is split between "what the body stores" and "what leaves as heat from the work of digesting and absorbing". The share F is a value actually measured for each nutrient. |
Energy used for digestion (kcal) οΌ Calories in the meal (kcal) Γ typical share
| Typical share | It varies with what you eat, but here we use about 10% as a guide |
| Energy used for digestion (kcal) | 800 Γ 0.1 οΌ 80 |
| Result | About 80 kcal is used by digestion itself |
| Energy used for digestion (kcal) | 800 Γ 0.2 οΌ 160 |
| Difference from β‘ (kcal) | 160 β 80 οΌ 80 |
| Result | About 160 kcal is used by digestion, about 80 kcal more than in β‘ |
The share of energy used by digestion itself is said to vary with the type of food. In general, foods rich in protein are said to tend to use a higher share of energy for digestion than carbohydrates or fats.
β» The shares here are commonly quoted guide values. Actual values are said to vary with the type of food and from person to person.
High school+How the Nerve Cells That Keep Us Awake Work
In a part of the brain called the hypothalamus, there are nerve cells that make a substance called "orexin", which is thought to play an important role in keeping us awake. Studies in animals have reported that when blood sugar rises, these nerve cells become less active.
UniversityPost-Meal Sleepiness Seen from Neuroendocrinology
In neuroendocrinology, researchers study how changes in blood sugar and hormones such as insulin after a meal affect the balance of various neurotransmitters in the brain and so adjust the balance between waking and sleeping. Post-meal sleepiness is thought to be a complex phenomenon involving several hormones and neurotransmitters. The orexin nervous system is the main player on the side that keeps us awake.
π Going further: Orexin (Japanese Wikipedia)
ResearchWhat Is Still Unclear
- Why sleepiness after a meal differs so much between people is a topic still being studied in neuroendocrinology.
- The link between "blood sugar spikes", where blood sugar rises and falls sharply, and lifestyle-related diseases is also considered an important research question.
- How far changing the order of eating or the pace of a meal can ease post-meal sleepiness and improve well-being is still being investigated in nutrition science.
Even the sleepiness after an everyday meal hides a rich topic, where neuroscience and nutrition science meet and research continues.
Links to Textbooks (by Level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Middle school | Science: digestion and blood | Basic terms: blood sugar, autonomic nerves |
| High school | Basic biology: energy metabolism | Calculating the energy used in digestion |
| High school+ | Biology: how the brain works (advanced) | The orexin system and wakefulness |
| University | Neuroendocrinology | Regulating wakefulness with hormones and neurotransmitters |
| Research | Neuroendocrinology and nutrition science (ongoing research) | Explaining individual differences, the link to blood sugar spikes, and research on eating methods |
- Explanations of blood sugar and the autonomic nervous system in physiology textbooks and materials.
- Research reviews on the orexin system and the regulation of wakefulness in neuroscience.
- Explanations of diet-induced thermogenesis in nutrition science materials.
- Research reviews on hormonal changes after eating and their link to sleepiness in neuroendocrinology.
- Explanations of ways of eating that curb rises in blood sugar in nutrition science materials.
β» The energy shares and the strength of sleepiness are said to vary with what you eat and from person to person.
β»This article is a general science explanation. If strong sleepiness or poor health continues, don't decide for yourself; see a medical professional.