Eating turkey causes drowsiness because it's high in the amino acid tryptophan which produces serotonin.
While turkey does contain tryptophan, chicken and many other foods contain similar or higher amounts. Post-Thanksgiving sleepiness is more likely caused by eating a large meal overall, alcohol consumption, and tradition rather than turkey specifically. Tryptophan in food has minimal effect on brain serotonin levels due to competition with other amino acids.
Reception
Sources
- Turkey and Tryptophan - NIH PRIMARY
- Snopes: Turkey Sleepiness REFERENCE