What is one possible explanation as to why tryptophan makes you sleepy?

Congress is controlled by the democrats. president yaeger, a republican, takes several actions that are opposed by the democrats in congress. first, the president introduces a new budget that cuts most of the government's social programs and gives tax breaks to the wealthy. next, president yaeger negotiates a trade treaty with a nation accused of human rights abuses. then, the president gives pardons to three republican senators who have been convicted of crimes.