Final answer:
The pineal gland secretes the hormone melatonin, which regulates circadian rhythms like the sleep-wake cycle and has a role in immune function and antioxidative protection of the CNS.
Step-by-step explanation:
The major hormone secreted by the pineal gland and involved with circadian and seasonal cycles is melatonin. This hormone plays a crucial role in regulating biological rhythms, including the sleep-wake cycle. The production of melatonin is stimulated by darkness and inhibited by light, which is essential for aligning our circadian cycles with the natural day-night pattern. During the night, or dark photoperiods, the pineal gland increases the production of melatonin, which can induce drowsiness and help regulate sleep.
Furthermore, melatonin has an effect on immune function and in some mammals, it can influence reproductive activities by affecting the production and maturation of reproductive cells and organs. Its antioxidative properties also protect the central nervous system from harmful free radicals. The onset of jet lag, as an example, is attributable to the time it takes for melatonin synthesis to adjust to new light-dark cycles after traveling across time zones.