Final answer:
Melatonin is the natural hormone produced by the pineal gland that induces sleep and is regulated by the light-dark cycle, contributing to the maintenance of circadian rhythms.
Step-by-step explanation:
The natural hormone produced by the pineal gland that induces sleep is melatonin. The secretion of melatonin is closely linked to the light-dark cycle. During times of darkness, the pineal gland increases the production of melatonin, leading to drowsiness and helping to regulate the sleep-wake cycle. Conversely, exposure to bright light inhibits melatonin production and promotes wakefulness. This hormone is not only vital for sleep induction but also plays a role in circadian rhythms and biological rhythms such as eating habits, and it is an antioxidant that protects the central nervous system.