Thanksgiving is a holiday surrounded by tradition, whether it be cooking all day, playing a game of flag football, or enjoying an annual post-dinner nap. Most people attribute the drowsiness they feel after a large Thanksgiving meal to the turkey, but many scientists disagree. While there are many aspects of a Thanksgiving dinner that may spur on a nap, turkey is not the culprit.
Turkey is often blamed for the post-Thanksgiving coma because it contains high levels of tryptophan, one of the 20 naturally occurring amino acids. Once consumed, the body uses tryptophan to make a neurotransmitter called serotonin. Neurotransmitters are chemical messengers that make it possible for nerve cells to communicate with one another. Serotonin is linked to mood and sleep patterns, and may be responsible for promoting slow-wave sleep in humans. However, it’s not clear yet if serotonin actually causes drowsiness directly. Research conducted by neuroscientist Amita Sehgal of the University of Pennsylvania School of Medicine suggests that serotonin causes fruit flies to grow tired, but this effect hasn’t yet been proved in humans.
Moreover, even though turkey contains tryptophan, a key ingredient in serotonin production, eating turkey doesn’t necessarily cause increased serotonin levels. Neuropharmacologist Richard Wurtman of the Massachusetts Institute of Technology explains that it takes more than just eating turkey to activate tryptophan in the body and brain. This is because turkey contains many amino acids, and tryptophan is not the most prevalent. After eating turkey, these amino acids circulate in the bloodstream, but cannot cross the blood- brain barrier without the aid of transport proteins. Like people trying to fit on a crowded bus, these amino acids have to compete for transport proteins to carry them across the blood-brain barrier and into the brain. Because tryptophan is outnumbered by other amino acids, which all need the same kind of transport protein to access the brain, tryptophan often doesn’t make it that far.
So, what really makes you tired after a Thanksgiving meal?
Biologist H. Craig Heller of Stanford University outlines many reasons why your feast is often followed by a nap. Heller explains that eating a lot of anything– turkey or not– causes drowsiness because loading the stomach with protein and fat, and stretching the small intestine, induces sleepiness. When this occurs, more blood is sent to the gastrointestinal tract to aid digestion, and less is sent to the brain and muscles. Another reason is that eating large quantities can trigger the parasympathetic nervous system to kick into action. The parasympathetic nervous system works in opposition to the sympathetic nervous system, which causes the fight or flight responses in stressful situations. Therefore, instead of getting you pumped up, the parasympathetic nervous system will calm you down by slowing your breathing rate and conserving energy. Drinking alcohol and eating large amounts of carbohydrates doesn’t help with wakefulness either, and after a full day with all of your relatives, it’s not surprising you would need a nap.
Caroline Petrow-Cohen ’18