While scientists say that the tryptophan in turkey is probably not the source of holiday fatigue, a possible new role for tryptophan has recently been uncovered. It appears to affect our sense of trust.Here's bit of what Wikipedia has about tryptophan:
Tryptophan as a component of dietary protein is particularly plentiful in chocolate, oats, bananas,mangoes, dried dates, milk, yogurt, cottage cheese, red meat, eggs, fish, poultry, sesame, chickpeas, sunflower seeds, pumpkin seeds, spirulina, and peanuts. It is also found in turkey at a level typical of poultry in general.
One widely-held urban myth is that heavy consumption of turkey meat (as for example in a Thanksgiving feast) results in drowsiness, which has been attributed to high levels of tryptophan contained in turkey. While turkey does contain high levels of tryptophan, the amount is comparable to that contained in most other meats. Furthermore postprandial Thanksgiving sedation may have more to do with what is consumed along with the turkey, and in particular carbohydrates, rather than the turkey itself.
... in particular, the common American post-Thanksgiving dinner drowsiness, may be the result of a heavy meal rich in carbohydrates, which, via an indirect mechanism, increases the production of sleep-promoting serotonin and melatonin in the brain.