**Core Concept**
Nightmare is a parasomnia characterized by vivid, often frightening dreams that occur during the rapid eye movement (REM) stage of sleep. This phenomenon is closely related to the brain's neurotransmitter regulation, particularly the balance between acetylcholine and norepinephrine.

**Why the Correct Answer is Right**
Nightmare is typically seen in sleep disorders such as narcolepsy, sleep apnea, and restless leg syndrome. The REM sleep stage is when most dreams occur, and the brain's neurotransmitter imbalance can lead to vivid, disturbing dreams. The correct answer is related to the sleep disorder that is most commonly associated with nightmares.

**Why Each Wrong Option is Incorrect**
**Option A:** This is incorrect because sleep deprivation is more commonly associated with decreased REM sleep and reduced dream intensity, not increased nightmares.
**Option B:** This is incorrect because insomnia is more commonly associated with difficulty falling asleep or staying asleep, rather than nightmares.
**Option C:** This is incorrect because sleepwalking (somnambulism) is a different type of parasomnia that typically occurs during non-REM sleep stages.

**Clinical Pearl / High-Yield Fact**
A mnemonic to remember common parasomnias is "BEARS": Bedwetting (nocturnal enuresis), Eating (sleep-related eating disorder), Awakening (sleep talking), Restless Legs (restless leg syndrome), and Sleepwalking (somnambulism).

**Correct Answer: D. Narcolepsy**