Found in protein-rich foods, this amino acid is converted in the brain into 5-HTP and then serotonin, a neurotransmitter that affects mood and sleep. Manufacturers of L-tryptophan supplements thus tout them to treat insomnia, boost mood, and alleviate depression, despite limited and inconsistent evidence of benefit in human trials. Some research has found that the supplement helps people fall asleep faster, but doesn't help them sleep longer. More studies are needed to see whether the supplements improve mood or have other benefits.
There are safety concerns too. Though banned in 1989 by the FDA after an outbreak of debilitating eosinophilia-myalgia syndrome (EMS)—which was traced to a Japanese company that apparently introduced an impurity during manufacture—the supplement has returned to market. But that doesn't mean it's a good idea to take it. And just because tryptophan is a food constituent, that doesn't mean it's safe in isolation and in large doses. In fact, a few more cases of EMS have been associated with the supplements over the past decade or so, not attributed to impurities.
Our take: Don't take L-tryptophan (or closely related 5-HTP, also sold as a supplement). The pills may pose health risks, besides the risk of contamination. And a large intake of any single amino acid may interfere with the absorption or utilization of other amino acids.
See also: 15+ Sleep Remedies