I noticed that REM Caps contain 5-HTP, which I heard is somewhat similar to the amino acid tryptophan. What's different about 5-HTP?

5-HTP (5-Hydroxytryptophan) is an extract from a natural legume, Griffonia simplicfolia, not from the fermented form of the isolated amino acid, tryptophan. 5-HTP is actually a metabolite of the amino acid tryptophan, a direct precursor of the neurotransmitter serotonin. Tryptophan is also a precursor for serotonin but is two metabolic steps away from serotonin.