Thanksgiving has many myths that go along with it. Upon googling the topic, the first cite I came across was as follows.
The myth I found most interesting was listed as their “Myth #8”. It stated that turkey makes people drowsy. This cite stated that turkey contains tryptophan, which your body uses to create serotonin. Serotonin calms the brain and “plays a key role in sleep” according to this cite.
With all of that said, most people would then assume that turkey would make you sleepy.
They thing about tryptophan is that it only makes you sleepy if you have an empty stomach, there are not many other amino acids present in you, or many protein. Turkey is a source of protein and the tryptophan in it is not eaten on an empty stomach (there is now turkey in you), and therefore turkey does not cause drowsiness.
I presume that most people feel sleepy after eating turkey because they are full. They have just eaten so much food and just want to relax. They’ve waited a year for a Thanksgiving to come again and now they are satisfied and want to sleep. It is all mental.