An in character term for an out-of-character phenomenon, wherein a character suddenly stops talking or acting ("falls asleep") for a long period of time, often because the player has not logged in or done anything with the character in a while.
In some cases, the sleeping sickness is terminal as the character dies when the players account is deleted due to inactivity. (This happens roughly 21 days after the last login.) To avoid this fate should you need be absent for a period of time, Set your account status to On Leave. This can be found by clicking on the Change button in the player information section of the main page. (The maximum amount of time you can set is 90 days.)
This is not to be confused with any actual game programmed disease.