The pineal body secretes which hormone that maintains the body's internal clock, the 24-hour wake-sleep cycle, and regulates the onset and duration of sleep?
a. oxytocin
b. calcitonin
c. melatonin
d. cortisol

The correct answer to the question: The pineal body secretes which hormone that maintains the body´s internal clock, the 24-hour wake-sleep cycle, and regulates the onset and duration of sleep?, would be, C: Melatonin.

Explanation:

The process of sleep and wakefulness, also called the circadian cycle, its a pretty complex system that is controlled by several parts of the brain, but most importantly, structures of the diencephalon (vital is the hypothalamus), the pineal gland, and the stem of the brain. All these structures, and some others, respond to changes both in light perception by the eyes and other senses, heat, and homeostatic balances. These factor play a vital role in telling the human body that it is time for sleep, or to awaken. The pineal gland, as part of this melody, is essential, as it secretes the hormone, melatonin, in response to light changes. Melatonin will balance out the process of sleep, when it is secreted in high quantities by the pineal gland, and wakefulness, when it is no longer secreted in response to the presence of light. This is what is called the circadian cycle.