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Tryptophan. For years, that was our excuse for sneaking away from the table to "test out the guest pillows" after Thanksgiving dinner. Tryptophan had become the dinner guest's kryptonite. Then we started hearing that we weren't actually getting that much tryptophan in a couple of slices of turkey--no more than the same serving of roast chicken breast or tofu, and not enough to explain our overpowering drowsiness. And when we thought about that, it made sense: We don't expect to spend an afternoon comatose after eating a turkey and cheese sandwich from the deli. So what's making us so tired on Thanksgiving?

Lots of things, it turns out, including what we did the night before the big feast, how many helpings we ate of Nana's marshmallow sweet potatoes, what we drank with our meal, and what activities we have planned for Thursday evening. (See the nutrition blog at Boston.com for the full explanation, as well as a unique theory from a professor who has researched napping).

This Thursday, when you start feeling snoozy after dinner, give the turkey a break. The poor thing has been through enough already.