L-tryptophan is an amino acid. The brain lowers tryptophan into serotonin, the happiness hormone. In the absence of this amino acid, serotonin deficiency occurs, which causes mood drops, depression, and insomnia. Serotonin is responsible for impulsive behaviour and sexual needs. The effect of this amino acid on depressive states was confirmed when its deficiency in the body was artificially induced during the study. Most of the respondents worsened their depressive symptoms, which were gradually alleviated as a result of taking tryptophan supplements. Tryptophan is also an ally of restorative sleep. Tryptophan also helps to build pure muscle mass by stimulating cell growth, which contributes to the improvement of physical fitness. After conversion into melatonin, it increases the body’s resistance.