Will eating turkey really make you sleepy? Tryptophan is an amino acid that is the precursor to serotonin, a neurotransmitter that can make you sleepy. There is no conclusive research linking tryptophan to sleepiness but there are indications that the increased consumption of carbs, such as pumpkin pie, may allow the tryptophan to enter the brain at a higher rate. Other good sources of tryptophan are milk, pork loin and swiss cheese. So eat your turkey leftovers and prepare for a nice afternoon nap today!