Which hormone produced by the pineal gland is associated with sleepiness?

The hormone produced by the pineal gland that is associated with sleepiness is melatonin. Melatonin plays a crucial role in regulating the sleep-wake cycle, also known as the circadian rhythm. Its production increases in response to darkness and decreases with exposure to light, signaling to the body that it is time to prepare for sleep. This is why melatonin is often referred to as the hormone of darkness, as it helps promote feelings of sleepiness and can aid in the transition to sleep.

Other hormones, like cortisol, serotonin, and dopamine, have different primary functions and are not primarily linked to sleep regulation. Cortisol, for instance, is more associated with stress responses and energy regulation. Serotonin is a neurotransmitter that affects mood and is a precursor to melatonin, but it is not directly responsible for sleepiness. Dopamine is primarily linked to reward and pleasure pathways in the brain. Therefore, melatonin is specifically recognized for its role in inducing sleepiness and promoting healthy sleep patterns.