Sleepers are how the people in the Magience world refer to Players. Time in the game world flows much more slowly compared to outside of it; 1 hour outside of Magience is equal to 1 day within it. So, when someone logs off to go to work or just doesn't play for a few days, their Character would be missing for quite some time within the game world's continuity.
To explain this, Characters are called Sleepers. When someone logs off, their Character's body cannot be seen, moved, harmed or altered in any way. A protective field surrounds them making them like ghosts. During this time they can be sensed with certain abilities or spells and, in some cases, smelled. But no magic exists to let someone touched a logged out Sleeper.
Then, when the Player logs back in, the body becomes tangible again in the exact location where the Character was when they logged off.
Someone can awaken as a Sleeper at any time in their life (explaining why people go from a "normal life" to "suddenly" being a Sleeper), often preceeded by a brief amount of time in which the person's memories begin to fade or vanish completely. Sleepers are considered to be "chosen" for greatness, and have great potential within them; they can grow to amazing strength and ability, often becoming famous and important figures in society. In this way, their ability to level up and learn skills indefinitely is explained, as well as the common confusion and lack of knowledge a Player exhibits when starting the game and first becoming their Character.
No one's entirely sure how the name "Sleeper" ended up being assigned to these people, since they're also listed in ancient records as Children of Fate. There are, however, many theories.
Most of them are wrong.