The circadian rhythm is a natural, internal process that regulates the sleep-wake cycle and other physiological functions of higher animals, including humans. It is primarily controlled by the hormone melatonin, making option B the correct answer.

Melatonin is a hormone produced by the pineal gland in the brain. It is synthesized and released in response to darkness and suppressed by light. Melatonin production follows a circadian rhythm, with levels increasing in the evening and peaking during the night, promoting sleepiness. As morning approaches and light levels increase, melatonin secretion decreases, allowing wakefulness.

To understand how melatonin regulates the circadian rhythm, let's break down the process:

1. Suprachiasmatic nucleus (SCN): The SCN is a region in the brain's hypothalamus that acts as the master clock for the circadian rhythm. It receives input from the eyes, which detect light and send signals to the SCN.

2. Light detection: Light exposure inhibits the production of melatonin. Photoreceptors in the retina of the eyes detect light and send signals to the SCN.

3. SCN regulation: The SCN receives input from the eyes, which helps synchronize the circadian rhythm with the external light-dark cycle. It then sends signals to various areas of the brain and body to coordinate physiological processes.

4. Melatonin production: In the absence of light, the SCN signals the pineal gland to produce and release melatonin. Melatonin levels gradually increase in the evening, reaching peak levels during the night.

5. Sleep-wake regulation: Melatonin acts on receptors in the brain to promote sleepiness and regulate the sleep-wake cycle. It helps initiate and maintain sleep during the night. As morning approaches and light levels increase, melatonin secretion decreases, promoting wakefulness.

In summary, melatonin plays a crucial role in regulating the circadian rhythm by promoting sleepiness and synchronizing the sleep-wake cycle with the external light-dark cycle. Its production and secretion are controlled by the suprachiasmatic nucleus in response to light input from the eyes. The circadian rhythm helps maintain a regular sleep schedule and influences various physiological processes throughout the day.