Serotonin, an important brain neurotransmitter, is key in the regulation of appetite, mood, and melatonin production. The presence of serotonin in the brain is associated with a balanced emotional state. This is achieved in part by decreasing the activity of certain excitatory hormones, including dopamine and noradrenaline. Serotonin also acts as a satiety signal in the brain, thereby naturally regulating food intake. Additionally, as a precursor of melatonin, serotonin is involved in regulating sleep patterns. Serotonin is unable to cross the blood-brain barrier and is therefore synthesized in the brain. Tryptophan, an essential amino acid, is a precursor for the synthesis of serotonin. Tryptophan crosses the blood-brain barrier and is converted to L-5-hydroxytryptophan, which in turn is converted into serotonin. As a metabolic intermediate in the conversion of tryptophan into serotonin, 5-HTP can also serve as a precursor of serotonin. Although St. John's Wort has a variety of traditional uses, extracts of this flowering perennial are best known for their mood balancing properties. St. John's Wort extract is widely recognized for its positive, supportive effects on mental and emotional function. As co-enzymes, the B vitamins are essential components in most major metabolic reactions. They play an important role in energy production, including the metabolism of lipids, carbohydrates, and proteins. B vitamins are also important for blood cells, hormones, and nervous system function. As water-soluble substances, B vitamins are not generally stored in the body in any appreciable amounts (with the exception of vitamin B-12). Therefore, the body needs an adequate supply of B vitamins on a daily basis.