As _____ accumulates in the body, a person will begin to feel sleepy.

• A. melatonin
• B. dopamine
• C. endorphins
• D. insulin

Answer: (A)

Verified step by step guidance

1. Identify the function of each substance listed in the options: melatonin, dopamine, endorphins, and insulin.
2. Understand that melatonin is a hormone that regulates sleep-wake cycles and is associated with the feeling of sleepiness.
3. Recognize that dopamine is a neurotransmitter involved in reward and pleasure systems, not directly related to sleepiness.
4. Acknowledge that endorphins are neurotransmitters that act as natural painkillers and mood enhancers, not related to inducing sleep.
5. Note that insulin is a hormone that regulates blood sugar levels, not directly linked to sleepiness.