GB, YT2095, and shpj4 get a gold star. Eating a large meal diverts blood flow away from the brain to the stomach for digestion ... thus causing drowsiness. Tryptophan is generally only a drowsiness factor when ingested on an empty stomach.
Now - back to the original question:
Originally Posted by StirBlue
Do you think the preservatives in our foods cause drowsiness?
I would doubt it ... but if you have a specific preservative in mind I could look into it. Most of the food preservatives that I have looked into are naturally occuring and are more prone to cause hyperactivity than lethargy.