What hormone is primarily released by the pineal gland during sleep?

The hormone primarily released by the pineal gland during sleep is melatonin. Melatonin plays a crucial role in regulating sleep-wake cycles, also known as circadian rhythms. The production of melatonin is influenced by the light-dark cycle; it is typically produced in greater quantities when it is dark, signaling to the body that it is time to prepare for sleep. This hormone helps facilitate the transition to sleep and maintain sleep throughout the night.

In the context of the options provided, other hormones like insulin, cortisol, and adrenaline serve different functions within the body. For example, insulin regulates glucose levels, cortisol plays a role in stress response and metabolism, while adrenaline prepares the body for 'fight or flight' situations. Thus, their primary functions do not align with the regulation of sleep as melatonin does, making melatonin the correct answer.