**Core Concept**
The question is testing the student's understanding of the pharmacological properties of hypnotic drugs, specifically their mechanism of action and effects on sleep architecture. Zaleplon is a non-benzodiazepine hypnotic agent that acts on the GABA_A receptor to enhance the inhibitory effects of GABA, the primary inhibitory neurotransmitter in the brain.

**Why the Correct Answer is Right**
Zaleplon works by binding to the alpha-1 subunit of the GABA_A receptor, which is responsible for the receptor's activation. This binding enhances the receptor's affinity for GABA, increasing the duration of GABA's inhibitory effects on neuronal activity. As a result, zaleplon produces sedative and hypnotic effects without significantly affecting sleep architecture or having anticonvulsant or muscle-relaxing properties. Unlike other benzodiazepines, zaleplon has a selective action on the GABA_A receptor, which allows it to produce a more targeted effect on sleep.

**Why Each Wrong Option is Incorrect**
**Option A:** Buspirone is an anxiolytic agent that acts as a partial agonist at serotonin 5-HT1A receptors, but it does not have hypnotic properties and is not related to GABA_A receptor modulation.
**Option B:** Diazepam is a benzodiazepine that enhances GABA_A receptor activity, but it has significant anticonvulsant and muscle-relaxing properties, as well as a broader effect on sleep architecture.
**Option C:** Phenobarbital is a barbiturate that acts on the GABA_A receptor, but it also has significant anticonvulsant and muscle-relaxing properties, as well as a broader effect on sleep architecture.
**Option D:** (This option is the correct answer, so it is not included here.)

**Clinical Pearl / High-Yield Fact**
Zaleplon has a short half-life, which allows for a rapid onset of action and a shorter duration of effects, making it a useful option for patients who need to wake up during the night for work or other reasons. This property also reduces the risk of next-day sedation and cognitive impairment.

**✓ Correct Answer: D. Zaleplon**