Desiree is having difficulty sleeping. She has heard of a natural substance that might help her sleep, as it naturally occurs in the brain and apparently mediates the circadian rhythm and sleep. You are sure that she is referring to __________.

A) melatonin
B) serotonin
C) endorphins
D) norepinephrine

Melatonin

A hormone produced by the pineal gland that regulates sleep and wakefulness, often referred to as the "sleep hormone."

Circadian Rhythm

The physical, mental, and behavioral changes that follow a 24-hour cycle, primarily responding to light and darkness in an organism's environment.

Natural Substance

refers to any product or material found in nature, not artificially produced, often used in context with foods, medicines, and other products.