Serotonin

Also known as 5-hydroxytryptamine, it is a neurotransmitter derived from the amino acid tryptophan and is involved in regulating mood, sleep, emotional behaviour and aggression.

Reference:
Kruse, B. G. (1999). The lived experience of serenity: Using Parse’s research method. Nursing Science Quarterly, 12(2), 143-150. https://doi.org/10.1177/08943189922106576

Linda Fitzgerald, M. S. N. (1991). Serenity: Caring with perspective. Research and Theory for Nursing Practice, 5(2), 127.