Which sleep disorder is characterized by excessive daytime sleepiness?

Narcolepsy is characterized by excessive daytime sleepiness, which often presents as an overwhelming urge to sleep during the day. Individuals with narcolepsy may experience sudden sleep attacks that can occur at any time, regardless of how much sleep they have had at night. This condition is linked to a disruption in the regulation of sleep-wake cycles, causing individuals to fall into rapid eye movement (REM) sleep more quickly than those without the disorder.

The hallmark symptom of excessive daytime sleepiness distinguishes narcolepsy from other sleep disorders. For instance, while sleep apnea can also lead to daytime fatigue due to disrupted sleep, its primary feature is repeated breathing interruptions during sleep rather than excessive sleepiness alone. Insomnia, on the other hand, is characterized by difficulties in falling or staying asleep rather than excessive daytime sleepiness, mostly resulting from poor sleep quality. REM behavior disorder involves a lack of muscle atonia during REM sleep, leading to individuals acting out their dreams, but does not primarily cause excessive daytime sleepiness. Thus, narcolepsy is the most accurate choice for the condition defined in the question.