Final answer:
The hormone that promotes sleep is melatonin, which regulates the body's circadian rhythm and signals the body to prepare for sleep. Its production increases in the dark and decreases in light, and it is crucial for initiating sleep. The correct option is A.
Step-by-step explanation:
The hormone that promotes sleep is melatonin. Produced in the pineal gland within the brain, melatonin helps to regulate the body's circadian rhythm, essentially acting as a signal for the body to prepare for sleep. It is synthesized from the amino acid tryptophan and its production is influenced by the light-dark cycle, with levels naturally increasing as darkness falls and decreasing when it is light. The secretion of melatonin is crucial for initiating and maintaining sleep.
Other substances mentioned, such as L-tryptophan, are precursors to sleep-related compounds but are not hormones themselves. L-tryptophan is an amino acid that the body uses to produce serotonin, which can then be converted into melatonin. However, L-tryptophan itself does not promote sleep directly as a hormone. Progesterone and oxytocin are hormones involved in various physiological processes but are not primarily responsible for sleep promotion.
It is also worth noting that certain external substances like caffeine, which acts as an adenosine antagonist, can inhibit sleep by blocking the action of adenosine, a neurotransmitter that promotes sleepiness. By doing so, caffeine decreases sleepiness and promotes wakefulness.