Why do we get sleepy after eating?

The Facts, Myths and Magic


By Dr. Don Davis
Epicture1very year during the holidays, most of us temporarily suspend our health goals and start
eating.  It begins in November with a big bump on Thanksgiving
and cruises through December with a crescendo on Christmas and New Year’s.  Inevitably your weight will rise and you will be making familiar resolutions to exercise more and eat less.  In fact, most Americans report that they gain an average of 5 to 10 lbs. over the holidays.  Unfortunately, they won’t lose all that weight afterward, leading to an expanding mid-section year after year.

Those holiday binges of family fun result in more than increased weight in the long run like inflammation and blood sugar swings.  But what about the short run.  Why do we feel so tired and lethargic after our feast?


Myth #1- Increased L-tryptophan.


The most common explanation I hear during “big” dinners is that turkey has high levels of L-Tryptophan that turn into serotonin and then into melatonin that controls your sleep wake cycles.  So, the logic is that since we ate a bunch of turkey, we’re bound to get a bit mellow from the serotonin and sleepy from the melatonin.


Myth #2 – Blood drained from your brain.


Another explanation goes like this: Eating a large meal requires a lot of blood in the stomach and intestines and so this extra blood is shunted from the brain to the gut.  This then supposedly causes lower brain oxygen and we get sleepy


But is that true? Let’s take a look at the science of after-eating (post prandial) sleepiness.


First of all, what is L-Tryptophan?


L-Tryptophan is an essential amino acid.  This means that the body can’t make it so you must have it in your diet. These amino acids build proteins and tryptophan can be found in many foods like turkey, chicken, meat, cheese, fish and eggs.


It can convert into serotonin, a brain neurotransmitter used to elevate well-being, relaxation, improved sleep cycles and pain tolerance. Not only that, serotonin is used to make melatonin that promotes sleep.  But is that what is really causing the after-dinner nap?


Here is a chart with the concentration of tryptophan in each food. 


typtophan-foodsWe see from the chart that turkey is down on the list from several foods, and surprisingly has less tryptophan than chicken. So, what is causing that after meal drop?


When we eat cod that has about 3 times as much L-tryptophan we don’t feel any sleepier so Myth #1 is a bust.


Myth #2 is easy to bust.  The blood pressure and blood flow in the brain is a very, very constant 750ml per minute regardless if we are running, thinking, eating or sleeping.  Research has shown that no blood flow change occurs with big dinners and no food coma results.


So, what is the real answer? 


Well, the myths are wrong but have a bit of truth.  It’s true that L-tryptophan is increased with a high protein meal but it can’t get into the brain without some carbohydrate like pumpkin pie or dinner rolls.  L-tryptophan is a big amino acid and has to wait in line while the other smaller amino acids that came with the high protein dinner get into the brain. Carbs increase sugar which increases insulin.  Insulin then drives the other amino acids into the cells leaving L-tryptophan (bound to albumin) plenty of room to pass the blood brain barrier (BBB) and make you sleepy. Eating a small amount of carbohydrate before bed can also promote sleep as long as you have eaten plenty of L-tryptophan beforehand.


big-gutThis is the reason people that are pre-diabetic
with big waist girth (metabolic syndrome) seem to fall asleep before healthy people. They are hypersensitive to carbs and their insulin levels elevate much more, driving the other amino acids into the cells, leaving higher amounts of L-tryptophan in the brain.


One more theory!


When we eat, many newly discovered hormones like Orexin, Ghrelin, and Leptin are produced or inhibited that affect hunger, memory and alertness.  Theses hormones along with the stimulation of the vagus nerve that travels back to the brain to the sleep centers has been shown to affect alertness after holiday gorging.  The science is still being worked out and I’m sure we will be finding that these hormones will be center stage in a fuller understanding of hunger, eating and drowsiness.


Let me know what you think and if you were able to make it through dinner and beyond with crashing!




Debunking a myth: neurohormonal and vagal modulation of sleep centers, not redistribution of blood flow, may account for postprandial somnolence.

Med Hypotheses. 2004;63(5):778-82.

Bazar KA, Yun AJ, Lee PY



Leave A Comment


First Name
Email address