This amino acid is an essential metabolic agent for the 'feel good' neurotransmitter serotonin.
Serotonin is one of the most important chemicals in your brain, when you eat great food, get a massage, have sex, enjoy time with friends, or accomplish a personal goal it's serotonin that makes you feel great. As a part of serotonin production process, 5-HTP is an important cog of the chemical mechanism for happiness.
5-Hydroxytryptophan, itself occurs naturally in food in quantities too small to actually do us any good. The human body metabolizes 5-HTP from tryptophan, another amino acid which we find in eggs, spirulina, soybeans turkey and parmesan cheese. Supplement 5-HTP comes from a natural source, the seed of an african plant. Since it's so chemically important to a Limitless mind, it's highly recommended that you supplement 5-HTP, it can be bought as an over the counter diet supplement for $10-$30.
It's recommended that you take Vitamin B6 in combination with 5-HTP supplements as B6 ensures the healthy metabolism of 5-HTP.
Numerous studies (including double-blind, placebo-controlled clinical trials) have shown 5-HTP produces consistent results in combating depression. In fact a studies comparing 5-HTP to fluvoxamine show that 5-HTP is either equal to or more effective (without the dangerous side effects) than expensive prescription antidepressants like Prozac, Paxil, and Zoloft. It becomes even more effective at fighting depression when coupled with St. John's wort extract. It also has proven effect in combating primary fibromyalgia, anxiety, binge eating, Friedreich's ataxia, and insomnia as it helps normalize sleep patterns. There's even evidence that it helps block the obesity causing causing chemicals that motivate us to overeat.
On a personal note, 5-HTP is actually the 1st nootropic I ever used. When I was a teenager and would go to raves me and my friends would use 5-HTP the next day since ecstasy (MDMA) diminishes the serotonin levels.