5-HTP, or 5-hydroxytryptophan, is a naturally occurring amino acid that is a precursor to the neurotransmitter serotonin, which is responsible in part for mood, hunger and sleep. 5-HTP works by increasing the amount of serotonin produced in the brain. It can act as an antidepressant in moderate doses, from around 50-100 milligrams, and as a sleep aid in doses above 100 milligrams. Also, 5-HTP can suppress appetite and act as a cognitive enhancer, or nootropic.