The Origins of Food Coma - It's after lunch and boy am I sleepy. The question of the hour is whether this sudden urge to close my office door and nap is attributable to a late night out at Mike & Mike's Day After or to the firm's annual Thanksgiving luncheon. Of course there was turkey, and many have cited the urban legend about tryptophan. While this precursor to serotonin could conceivably have a narcotic effect on an empty stomach, most experts seem to agree that the more likely cause is overeating "which pulls all the blood away from your brain as your stomach begins the arduous task of digesting an overload." Combined with staying out 'til 1 a.m. on a school night, sounds like a deadly mix.