What is characterized by difficulty falling or maintaining sleep?

Insomnia Disorder is defined by difficulties in falling asleep, maintaining sleep, or obtaining restorative sleep, which leads to significant distress or impairment in functioning. The symptoms include trouble initiating sleep, waking up frequently during the night, or waking too early and not being able to go back to sleep. These sleep disturbances can cause a variety of negative consequences, such as daytime fatigue, mood disturbances, or difficulties with concentration and performance.

Unlike Hypersomnolence Disorder, which involves excessive daytime sleepiness despite prolonged sleep, or Narcolepsy, characterized by recurrent episodes of irresistible sleep, Insomnia Disorder specifically focuses on the challenges associated with sleep initiation and maintenance. Breathing-Related Sleep Disorders involve sleep disruptions due to respiratory issues, but they do not primarily describe the problem of falling or staying asleep as seen in Insomnia Disorder. Therefore, the distinctive feature of insomnia—which is the struggle with sleep initiation and maintenance—solidifies its designation as the correct answer.