Which hormone is primarily regulated by the pineal gland?

The hormone primarily regulated by the pineal gland is melatonin. This gland, located in the brain, synthesizes and secretes melatonin in response to darkness, playing a crucial role in regulating the sleep-wake cycle and circadian rhythms.

Melatonin levels typically rise in the evening, inducing sleepiness, and fall in the morning, promoting wakefulness. This hormone helps signal the body when it is time to rest, thereby influencing various physiological processes related to sleep. The regulation of melatonin by the pineal gland is particularly important in responding to environmental light changes, which help orchestrate a regular sleep pattern aligned with day and night cycles.

On the other hand, insulin is produced by the pancreas, thyroxine is produced by the thyroid gland, and cortisol is synthesized by the adrenal glands, all of which function independently of the pineal gland's influence. These hormones are involved in different bodily functions such as metabolism, growth, and stress response, but they do not play a direct role in the regulation of melatonin or circadian rhythms.