What hormone is primarily responsible for regulating sleep-wake cycles?

Melatonin is primarily responsible for regulating sleep-wake cycles, making it the correct choice in this context. This hormone, produced by the pineal gland in the brain, plays a crucial role in signaling the body when it is time to sleep. Its production is influenced by light exposure; levels typically rise in the evening as it gets darker, promoting feelings of sleepiness, and decrease in the presence of light, which helps the body wake up.

Cortisol, often known as the stress hormone, typically peaks in the early morning to help promote alertness and decrease throughout the day. Although it plays a significant role in several bodily functions, it is not primarily focused on sleep-wake regulation.

Serotonin is a neurotransmitter involved in various functions including mood regulation, appetite, and digestion, but it also influences sleep; however, it is not the main hormone that directly regulates sleep-wake cycles.

Adrenaline, also known as epinephrine, is primarily involved in the body’s fight-or-flight response and has an energizing effect, making it counterproductive to sleep.

The focus of melatonin on circadian rhythms and its direct regulation of sleep makes it the key hormone associated with sleep-wake cycles.