Which of the following conditions is characterized by episodes of excessive sleepiness during the day?

The condition characterized by episodes of excessive sleepiness during the day is narcolepsy. Narcolepsy is a neurological disorder that affects the brain's ability to regulate sleep-wake cycles, leading to sudden and uncontrollable bouts of sleep during the day. Individuals with narcolepsy may experience cataplexy, sudden muscle weakness triggered by strong emotions, in addition to excessive daytime sleepiness.

Other conditions listed, such as insomnia, involve difficulty in falling or staying asleep, which does not directly cause excessive daytime sleepiness but potentially results from sleep loss. Sleep apnea is characterized by interruptions in breathing during sleep, which can disrupt sleep quality, but the primary symptom is not the excessive sleepiness itself but rather the breathing disturbances. Restless leg syndrome primarily involves uncomfortable sensations that create an urge to move the legs, often disrupting sleep but not leading to the characteristic excessive daytime sleepiness seen in narcolepsy. Thus, narcolepsy is distinct in its definition by the occurrence of excessive daytime sleep episodes.