What condition is characterized by an inability to fall asleep or stay asleep?

The condition characterized by an inability to fall asleep or stay asleep is insomnia. Insomnia is a sleep disorder that can manifest in various ways, including difficulty initiating sleep, maintaining sleep, or waking up too early and being unable to return to sleep. This can lead to daytime fatigue and decreased functioning, impacting one's overall quality of life. Individuals suffering from insomnia may find that their thoughts race at night, or they may feel anxious about not sleeping, which can exacerbate the problem.

In contrast, sleep apnea involves interruptions in breathing during sleep and is characterized by loud snoring or gasping for air, not primarily by the difficulty in falling or staying asleep. Sleepwalking involves walking or performing other activities while in a state of sleep but does not specifically relate to falling asleep or staying asleep. Narcolepsy is a neurological disorder that affects the control of sleep and wakefulness, leading to sudden sleep attacks, but again, it does not specifically describe the inability to stay asleep through the night. Thus, insomnia directly addresses the symptoms related to sleep initiation and maintenance, making it the correct choice.