Turkey is a protein and it contains an fairly high amount of the Amino Acid called 'Tryptophan.' Tryptophan is an essential Amino Acid, not one your body makes. You have to consume it.
Key information to know about where this myth comes from is as follows, just two points to know.
1st point: Tryptophan is used by your body to make Serotonin. Serotonin is part of what regulates your mood.
2nd point: Our body makes Melotonin from Serotonin. Melotonin is the hormone that makes us sleepy.
So you can see the importance of consuming Tryptophan in your diet. If your Tryptophan levels are not good then your mood can be affected. Low serotonin levels are linked with depression and anxiety.
Melotonin and serotonin are vitally important for your sleep and relaxation levels or the ability to relax.
This is where the half truth around Turkey making you sleepy comes from.
Does that mean to eat lots of Turkey and you will feel both great and sleepy? Well, actually no, and actually in fact Tofu contains more Tryptophan than Turkey.
To sum up on this myth about Turkey making you sleepy:
It is true that eating Turkey gives you some Tryptophan. It is also true that Tryptophan makes you feel relaxed and it is part of what makes Melotonin which is the sleepy hormone.
However, eating Turkey in itself is unlikely to be the cause of falling asleep after your Turkey dinner! It is far more likely to be simple, tiredness, overindulgence and excess carbohydrates which has the real sedating effect.