Which of the following is NOT a stimulant that could impact sleep?

Melatonin is not a stimulant and is actually known for its role in regulating sleep cycles. It is a hormone produced by the pineal gland in the brain and helps signal to the body that it is time to sleep, contributing to the natural sleep-wake cycle. Unlike stimulants, which promote alertness and can hinder the ability to fall asleep or remain asleep, melatonin is used therapeutically to enhance sleep quality and help manage sleep disorders. This distinct function makes it different from the other options listed, which are known to stimulate the central nervous system and can disrupt sleep patterns.