In the human body, the 'Pineal gland' is often referred to as the 'biological clock' because it secretes: MCQ with Answer and Explanation

The pineal gland, a small endocrine gland located in the epithalamus of the brain, is often called the 'biological clock' or the 'third eye' in some lower vertebrates. Its primary function in humans is to secrete the hormone melatonin. Melatonin production is regulated by the light-dark cycle (circadian rhythm); it is produced in higher amounts during the night and suppressed by light. Melatonin helps regulate sleep-wake cycles, seasonal reproductive functions, and metabolism.