Roughly half of all people with Narcolepsy also suffer from Cataplexy.
Cataplexy is a sudden loss of muscle tone, often brought on by strong emotions such as laughter or anger.
Severity can range from weakness in the face (sometimes seen as a loose jaw, or tongue hanging out), knees, arms or neck and slurred speech, through to its most severe form where a sufferer can collapse to the ground. Even in a complete collapse situation, the sufferer is completely concious and aware of what people are saying, but are effectively paralised.
The episodes usually last no more than a few minutes, but can be extremely frightening and dangerous if they happen in the wrong place.