While a character can be played on its own, players can group with others to tackle more challenging content. Most end-game challenges are designed in a way that they can only be overcome while in a group. In this way, character classes are used in specific roles within a group. World of Warcraft uses a "rested bonus" system, increasing the rate that a character can gain experience points after the player has spent time away from the game. When a character dies, it becomes a ghost—or wisp for Night Elf characters—at a nearby graveyard. Characters can be resurrected by other characters that have the ability or can self-resurrect by moving from the graveyard to the place where they died. If a character is past level ten and they resurrect at a graveyard, the items equipped by the character degrade, requiring in-game money and a specialist NPC to repair them. Items that have degraded heavily become unusable until they are repaired. If the location of the character's body is unreachable, they can use a special "spirit healer" NPC to resurrect at the graveyard. When the spirit healer revives a character, items equipped by the character at that time are further degraded, and the character is significantly weakened by what is in-game called "resurrection sickness" for up to ten minutes, depending on the character's level. This "resurrection sickness" does not occur and item degradation is less severe if the character revives by locating its body, or is resurrected by another player through spells or special items.