Narcolepsy is a sleep disorder that causes disabling daytime sleepiness and other symptoms. Narcolepsy is related to the dreaming period of sleep called REM (rapid eye movement) sleep. With narcolepsy, though, REM periods can occur throughout the day. In addition to drowsiness that doesn’t improve, narcolepsy may cause brief uncontrollable moments of sleep, or “sleep attacks,” without warning. Another daytime hallmark of narcolepsy is sudden loss of muscle control, or cataplexy. This can be a slight feeling of weakness or total body collapse. It can last from seconds up to a minute. Cataplexy is related to the muscle immobility, or “paralysis,” that is part of REM sleep. It is often triggered by emotions or fatigue.