The hormone that regulates the sleep-wake cycle in humans is called melatonin. Produced by the pineal gland in response to darkness, melatonin helps signal the body that it is time to sleep. Its levels rise in the evening and fall in the morning, aligning with environmental light cues. Disruptions in melatonin production can lead to sleep disorders, emphasizing its importance in circadian rhythms and overall health. Maintaining adequate melatonin levels is vital for achieving restorative sleep and effective functioning.