Players receive action points as time passes, which are used up any time they move, fight, or undertake any other sort of activity. One action point is gained every half-hour of realtime, up to a maximum of fifty points. When a character's action points drop to zero or lower, that character is unable to perform any action until they gain further points, survivors fall asleep and zombies' brains cease to function. Should a survivor run out of action points whilst in an unsecured area, they are at great risk of attack by zombies during sleep. Players must make sure that their survivor characters are in a safe place before logging out, encouraging quick raids from fortified safe houses. The use of Action Points as a limit on character actions is similar to the systems of several other free MMORPGs such as Kingdom of Loathing. As players damage their opponents or perform certain other actions, their experience points increase. While survivors can also gain experience from performing a variety of actions, such as healing other characters or repairing damage to buildings, zombies can only gain experience points by attacking other characters or objects in the game world. When players gain enough experience points, they can buy new skills. A character can only buy survivor skills while alive, and only zombie skills while dead; although skills of one type are not lost if the character dies or is revived, most survivor skills cannot be used by zombies, and vice versa. Survivor characters can effectively learn the skills of all survivor classes, not just the class they begin with, though skills outside of their chosen class can be more expensive. For a long time, the highest level that a player could reach if all skills were purchased was 42, however due to the skill "Brain Rot" making it more difficult to get revives, many players opt to leave it unpurchased and as a result there are a greater number of players at level 41 as opposed to level 42. On October 14, 2008, a new zombie skill "Flesh Rot" was added, thus making it possible to reach level 43. With the new additions of the skills "Bellow" and "Scout safehouse" in August 2010, the highest attainable level is now 45. Survivors who die and return as zombies do not have to switch sides - they can gather at designated "revive points" scattered throughout