Doctor's Best Melatonin supplies melatonin, a naturally secreted substance in the body to help regulate circadian rhythm, often referred to as the ""internal body clock"", that tells our body when to sleep, rise, eat - regulating many physiological processes for the body and the brain to function normally. Certain factors may affect circadian rhythm such as individuals adjusting sleep-wake cycles with daily work schedule shifts or time zone changes and jet lags.