Tryptophan molecule. Computer model of a molecule of the amino acid tryptophan. Atoms are represented as rods and are colour-coded: carbon (green), hydrogen (white), nitrogen (blue) and oxygen (pink). Amino acids are the building blocks of proteins. Tryptophan is an essential amino acid, meaning that it cannot be synthesised by the body and so must be obtained from the diet. It is found in meat, poultry, dairy products and peanuts. In the body it is a precursor for serotonin (an inhibitory neurotransmitter), melatonin (a sleep-inducing hormone) and niacin (a b vitamin). Its ability to increase brain levels of serotonin and melatonin have led to it being used in supplement form as a sleep aid.