Question

A man has difficulty in sleep during the night. He has a habit of drinking coffee before bed time. What is the role of caffeine in wakefulness?

• A. Blocks adenosine action and causes wakefulness
• B. Activates locus coeruleus and causes wakefulness
• C. No role in maintaining wakefulness if taken 1 hr before sleep
• D. Activates histamine release and prevents sleep

Hint:

Caffeine is a competitive antagonist at the receptor of the brain's main sleep-promoting molecule. Which molecule?

Answer 1

The Correct Option is A

Step 1: Recall the sleep role of adenosine.
Adenosine accumulates in the brain during prolonged wakefulness and is a key endogenous sleep-promoting substance. Acting at A\(_1\) and A\(_{2A}\) receptors, it inhibits arousal-promoting neurons and increases "sleep pressure" (the drive to sleep that builds the longer one stays awake).

Step 2: Mechanism of caffeine.
Caffeine is a competitive antagonist at adenosine receptors. By blocking adenosine's action, it removes adenosine's inhibitory/sleep-promoting effect, so the arousal systems stay active and the person remains awake. This is precisely why coffee before bed disrupts sleep.

Step 3: Eliminate the distractors.
(B) Caffeine does not act primarily by activating the locus coeruleus; its main, well-established mechanism is adenosine-receptor blockade. (C) Is wrong because caffeine has a half-life of several hours, so coffee taken even an hour (or more) before bed certainly impairs sleep. (D) Caffeine's wakefulness is not explained by triggering histamine release - again, the central mechanism is adenosine antagonism.

Final answer: A - caffeine blocks adenosine action, producing wakefulness.