Which hormone is primarily responsible for regulating sleep and biological rhythms?

The correct answer is melatonin, which plays a crucial role in regulating sleep and biological rhythms, often referred to as the circadian rhythm. Melatonin is produced by the pineal gland in response to darkness, signaling to the body that it is time to prepare for sleep. This hormone helps to regulate sleep-wake cycles, promoting feelings of sleepiness, and thus it is often associated with the body's natural sleep patterns and timing.

Additionally, melatonin helps to regulate other biological rhythms and processes, ensuring that they align with the day-night cycle. For individuals who experience disruptions in their circadian rhythms, such as shift workers or those suffering from jet lag, melatonin supplementation can help re-establish normal sleep patterns.

The other hormones listed, while important in various physiological processes, do not primarily regulate sleep in the same manner. Cortisol is related to stress responses and follows a diurnal pattern peaking in the morning and declining throughout the day. Adrenaline is tied to the body's fight-or-flight response and increases alertness rather than promoting sleep. Serotonin, although it contributes to mood regulation and can be a precursor to melatonin, does not directly regulate sleep and biological rhythms on its own.