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When a hearty meal induces drowsiness, it is because blood leaves the brain and goes to the stomach (gut). Surely you have heard your mother or your grandmother or somebody tell that old wives' tale, and maybe you believed it. It is nonsense, according to Hartmann et al in a report appearing in the current issue of Archives of General Psychiatry (31:394-397, 1974).
Complete hydrolysis of proteins yields some 20 different amino acids some of which are "essential" or "indispensable" in that they serve important functions but cannot be synthesized by the body. Among the indispensable amino acids derived from food is L-tryptophan, which, among other actions, in animals and man has been shown to induce sleep, presumably by raising blood levels of serotonin.
Accordingly, Hartmann et al undertook a sleep-laboratory study of the effects of L-tryptophan on sleep latency (the time from "lights out" until sleep) in a group
Hugh H. Hussey. Sleep Inducement by L-Tryptophan. JAMA. 1974;229(11):1474–1475. doi:10.1001/jama.1974.03230490062032