Which condition is characterized by excessive daytime sleepiness?

The condition characterized by excessive daytime sleepiness includes both narcolepsy and sleep apnea.

Narcolepsy is specifically known for sudden episodes of overwhelming sleepiness that can occur during the daytime. Individuals with narcolepsy may experience uncontrollable urges to sleep, which can significantly disrupt daily activities. This condition often includes symptoms such as cataplexy, hallucinations, and sleep paralysis, but excessive daytime sleepiness is a hallmark characteristic.

Sleep apnea, particularly obstructive sleep apnea, can also lead to excessive daytime sleepiness. In this condition, breathing repeatedly stops and starts during sleep, resulting in fragmented sleep and reduced quality. This disruption causes individuals to feel unusually tired during the day despite seemingly adequate hours of sleep.

Both conditions lead to significant daytime sleepiness, making option D the most accurate choice as it encompasses multiple disorders that share this common symptom.