No, it is not the turkey that puts you to sleep after that humongous Thanksgiving day meal–it’s the stuffing. Wait, what?! Stuffing does not have much tryptophan in it and it’s tryptophan that makes you sleepy!

That is true, the tryptophan from turkey is a necessary ingredient in post-meal Zzs, but it is not the only ingredient you need. You need the carbs. Let me quickly explain why:

Tryptophan is the precursor molecule for serotonin (^_^ neurotransmitter), which is the precursor for melotonin (sleepy time). In order to get melatonin from serotonin from tryptophan, you need tryptophan in your brain. Unfortunately for the turkey-only explanation of post-meal fatigue, tryptophan has a tough time crossing the blood-brain barrier which is what all amino-acids you consume through diet needs to cross in order to be used by your brain.

Tryptophan is transported across the blood-brain barrier via the Large Neutral Amino Acid Transporter. Usually the transporter is bombarded with other amino acids which have much higher concentrations in the blood than tryptophan, so not much tryptophan crosses into the brain. However, after a large meal, with lots of stuffing and carbohydrates, something special happens.

Consuming carbohydrates releases insulin. Insulin in the blood triggers your muscles to absorb large amino acids like leucine, but not tryptophan. With less of the other large amino acids in the blood (after getting absorbed into muscle) there is a much larger ratio of tryptophan to other stuff in your blood, so there is less competition, so to speak, for tryptophan to cross your blood brain barrier. Thus:

More carbs–>more tryptophan in your brain–>more serotonin and more melatonin–>sleepiness after a big meal.

By the way, this is the same mechanism that explains why you crave CARBS and SUGAR (hello, cookies, ice cream, and bread) when you’re feeling blue: you need to make serotonin!