Excessive daytime sleepiness: A neurological disorder in which there is a sudden recurrent uncontrollable compulsion to sleep. Excessive daytime sleepiness is also known as narcolepsy. The condition is often associated with: 1  Cataplexy -- a sudden loss of muscle tone and paralysis of voluntary muscles associated with a strong emotion. 2  Sleep paralysis -- immobility of the body that occurs in the transition from sleep to wakefulness. 3  Hypnagogic hallucinations -- pre-sleep dreams.