Which of the following molecules is involved in determining when we sleep?

• A. Insulin
• B. Thyroxine
• C. Adrenaline
• D. Melatonin

Answer: (D)

Verified step by step guidance

1. Step 1: Begin by understanding the role of melatonin in the body. Melatonin is a hormone produced by the pineal gland in the brain, primarily in response to darkness. It helps regulate the sleep-wake cycle, also known as the circadian rhythm.
2. Step 2: Compare melatonin's function to the other molecules listed in the question. Insulin is a hormone that regulates blood sugar levels, thyroxine is involved in metabolism and growth, and adrenaline is responsible for the 'fight or flight' response during stress.
3. Step 3: Recognize that none of the other molecules listed (insulin, thyroxine, adrenaline) are directly involved in regulating sleep. Melatonin is unique in its role in signaling the body when it is time to sleep.
4. Step 4: Understand the mechanism of melatonin release. Melatonin production increases in response to darkness and decreases in response to light, helping the body align its sleep patterns with the day-night cycle.
5. Step 5: Conclude that melatonin is the correct answer because it is the molecule directly involved in determining when we sleep, based on its role in regulating the circadian rhythm.