Final answer:
Insomnia is the sleep disorder marked by difficulty in falling or staying asleep, distinguishing it from other sleep disorders such as sleep apnea, narcolepsy, and parasomnias.
Step-by-step explanation:
The sleep disorder characterized by difficulty in falling asleep or remaining asleep is called insomnia.
Insomnia is the most common sleep disorder and it includes symptoms such as long delays in falling asleep after going to bed and waking up multiple times during the night with trouble returning to sleep.
This condition typically requires symptoms to persist for at least three nights a week for a minimum of one month to be diagnosed.
In contrast, other sleep disorders like sleep apnea involve pauses in breathing during sleep, narcolepsy triggers sudden sleep episodes during the day, and parasomnias such as sleepwalking or night terrors involve unwanted motor behavior or experiences during sleep.