Tryptophan is an essential amino acid that creates niacin, which is key in creating the neurotransmitter serotonin. Serotonin is found mostly in the digestive system and is a chemical that regulates your cat’s mood, sleep-wake cycles, level of arousal, and sensitivity to pain/stimuli. You can make the connection of how important a healthy amount of serotonin is for your cat’s behavior; with a deficit of serotonin you may see hyperactivity, aggression, hypersensitivity or restless sleep in your cat.
Tryptophan comes from meat sources. Many commercial pet foods include corn and other simple carbohydrates as a primary protein – and the problem with these is they lack tryptophan. Feeding a quality mostly-meat based food is key to your cat feeling well.
Read the ingredient labels and avoid:
Meat product
Meat by-product
Wheat
Corn
(Note: Brown rice is ok)

"Meat product" and "Meat byproduct" are NOT to be avoided if you're trying to get tryptophan into cats. Both should be expected to contain tryptophan; neither is vegetable in nature.