Melatonin (noun)

A hormone that regulates sleep-wake cycles and is naturally produced in the body by the pineal gland, which is activated by darkness and inhibited by light.

Origin:

Late 20th century: from mela- (from greek melas, melan- "black") + -tonin.

Examples:

1. Melatonin is a hormone that regulates sleep-wake cycles.
2. She took a melatonin supplement to help her sleep.
3. The melatonin levels in the body are influenced by light.
4. The melatonin levels in the body decrease as we age.
5. Melatonin is a natural sleep aid.