What condition is characterized by the inability to initiate or maintain sleep?

Insomnia is characterized by the inability to initiate or maintain sleep, which aligns with the definition of the condition itself. People with insomnia may struggle to fall asleep when trying to do so, wake up frequently throughout the night, or wake up too early and find it difficult to go back to sleep. This disruption can lead to daytime fatigue, mood disturbances, and difficulties with concentration.

The other conditions listed have different characteristics. Sleep apnea, for example, is primarily a disorder of interrupted breathing during sleep, leading to frequent awakenings, but not specifically about the initiation or maintenance of sleep. Sleepwalking involves engaging in activities while in a state of sleep, typically during deep sleep stages, and does not pertain to the ability to either initiate or maintain sleep. Narcolepsy is characterized by excessive daytime sleepiness and sudden sleep attacks, rather than issues with initiating or maintaining sleep during the night. Each of these conditions present unique challenges related to sleep but do not define insomnia's core issue of sleep initiation and maintenance.