What hormone is released by the pineal gland to promote relaxation and tiredness?

The hormone released by the pineal gland that promotes relaxation and tiredness is melatonin. Melatonin is a hormone that regulates sleep-wake cycles and is produced in response to darkness, signaling to the body that it is time to prepare for sleep. This hormone helps to establish circadian rhythms, making it crucial for maintaining healthy sleep patterns. The increase in melatonin levels as night approaches contributes to feelings of drowsiness and assists in inducing sleep.

In contrast, serotonin is primarily a neurotransmitter that influences mood and is involved in regulating various bodily functions, including sleep but it is not the hormone directly responsible for promoting relaxation and tiredness. Cortisol, known as the stress hormone, typically increases alertness and energy levels, promoting wakefulness rather than relaxation. Adrenaline is another stress hormone that prepares the body for a 'fight or flight' response, which further opposes the feeling of relaxation and tiredness. Thus, melatonin uniquely fulfills the role of signaling the body to relax and prepare for sleep.