Tryptophan, Trust and the Thanksgiving Turkey

Reading time: 3 – 5 minutes

After feasting on your Thanksgiving dinner today, a specific word may come to mind: tryptophan. Tryptophan is an amino acid found in high levels in turkey. The main reason for the association between tryptophan and drowsiness is that tryptophan is a chemical precursor to melatonin, a neurotransmitter that plays a role in regulating the circadian cycle, the regular changes in mental and physical characteristics that occur over the course of 24 hours. Indeed, purified tryptophan is a mild sleep-inducing agent [1].

Thanksgiving turkey

Tryptophan breaks down into at least three metabolites. The first is melatonin, which can lead to sleepiness. The second is niacin, a form of vitamin B. Tryptophan also breaks down into serotonin, a neurotransmitter that regulates agression, body temperature, appetite and mood. In a recent study presented at the Society for Neurosciences, Robert Rodgers, Ph.D. and his colleagues at Oxford University used game theory to study serotonin’s role in social interactions [2].

The study asked volunteers to play a two-person game called Prisoner’s Dilemma. In the game, people make choices that affect each other, either choosing to make a move that wins them money from the other player or a move that wins both players money. Under normal circumstances, players were found to cooperate about 75% of the time. However, by depleting levels of l-tryptophan, which in turn reduced levels of serotonin, researchers found a significantly decreased level of cooperation among players. The lowered levels of serotonin also effected players judgment of the social characteristics of others and they rated fellow players as less trustworthy.

According to Rogers, serotonin may [2]:

… play a role in modulating the cognitions that underpin dependable relationships with our social partners.

The research findings suggest that serotonin deficiency may impair continual cooperation and diminish the reward value of cooperative behavior.

Does all this mean we’re more trusting after a Thanksgiving day meal? Likely not. Scientists haven’t yet determined how increased levels of tryptophan affect trust. However, after a large meal of turkey, stuffing, gravy, cranberries, sweet potatoes and pumpkin pie, brain levels of tryptophan may actually decrease.

The transport system that’s used to take tryptophan to the brain is shared by a number of other amino acids and there’s competition between them. Thus, after a large meal, less tryptophan will reach the brain than on an empty stomach. In addition, this means that, contrary to popular belief, the tryptophan in turkey does not cause drowsiness.

So why are we so sleepy after a Thanksgiving meal?

It’s not unusual to feel sleepy after eating a lot, especially after consuming large amounts of high-carbohydrate foods. Indeed, consumption of high glycemic index foods has been shown to decrease the time it takes to fall asleep [3]. Additionally, it’s not unusual to feel sedated after a high-calorie meal. The best way to reduce drowsiness this Thanksgiving? Simple … eat less.


  1. Hartmann et al. L-Tryptophane and sleep. Psychopharmacologia. 1971;19(2):114-27.
    View abstract
  2. Research reveals brain areas for types of decision-making, shows how a brain chemical underpins social interaction. Society for Neuroscience. 2007 Nov 4.
  3. Afaghi et al. High-glycemic-index carbohydrate meals shorten sleep onset. Am J Clin Nutr. 2007 Feb;85(2):426-30.
    View abstract
About the Author

Walter Jessen is a senior writer for Highlight HEALTH Media.