Tryptophan (abbreviated as Trp or W) is one of the 20 standard amino acids. It is an essential amino acid in the human diet, meaning it must be obtained form food sources.

Tryptophan functions as a biochemical precursor for serotonin (which can then be converted to melatonin by enzymes in the body). Because of this tryptophan produces a calming effect or acts as a sleep-aid. 

Tryptophan is a found in most protein-based foods or dietary proteins. It is particularly plentiful in chocolate, oats, bananas, mangoes, dried dates, milk, yogurt, cottage cheese, red meat, eggs, fish, poultry, sesame, chickpeas, sunflower seeds, pumpkin seeds, spirulina, and peanuts.  

References

IUPAC-IUBMB Joint Commission on Biochemical Nomenclature. Nomenclature and Symbolism for Amino Acids and Peptides. Recommendations on Organic & Biochemical Nomenclature, Symbols & Terminology etc. Retrieved on 2007-05-17. ; Radwanski ER, Last RL (1995). "Tryptophan biosynthesis and metabolism: biochemical and molecular genetics". Plant Cell 7 (7): 921–34. doi:10.1105/tpc.7.7.921. PMID 7640526.