The smallest gland in the human body is the Pineal Body.

The Pineal Body, also known as the Pineal Gland, is a small endocrine gland located in the brain. It is often referred to as the "third eye" due to its shape and location. Although it is small in size, measuring only about 8 millimeters in length, it plays a significant role in regulating various functions in the body.

The Pineal Body is located in the center of the brain, between the two hemispheres, and is situated just above the brainstem. It is shaped like a pinecone, hence its name. Despite its small size, it is responsible for producing and secreting the hormone called melatonin.

The production and secretion of melatonin by the Pineal Gland are influenced by the amount of light exposure a person receives. This hormone helps regulate the sleep-wake cycle, also known as the circadian rhythm. During the day, when there is more light, the production of melatonin is inhibited, promoting wakefulness. At night, when light levels are low, the production of melatonin increases, promoting sleep.

In addition to regulating the sleep-wake cycle, the Pineal Gland is also involved in other physiological processes. It has been suggested to play a role in the regulation of mood, as well as in the onset of puberty. Additionally, the Pineal Gland has been associated with the regulation of reproductive hormones and the immune system.

In conclusion, while the Pineal Gland may be the smallest gland in the human body, it plays a crucial role in regulating various functions, particularly the sleep-wake cycle through the secretion of melatonin.