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A sleeper account is an account that has not been edited in a long time and essentially is dormant. Technically, a sleeper account is still enabled, and it is still possible to use it. However, any such use of an account must conform to Wikipedia guidelines, particularly those of sockpuppetry.
No known period without editing makes an account a sleeper account. Many editors take long, unannounced breaks from editing, but they may return to Wikipedia after a long absence of weeks, months, or even years. Provided the account is in good standing and that the same person is not deceptively operating another account, which may violate sock puppetry guidelines, there is no objection to resuming editing after a long absence. A large majority of auto-confirmed accounts on Wikipedia are inactive (sleeper) accounts.
However, it is also possible that a dormant account was created to become a sockpuppet account and used for unallowed purposes. This type of account usually performs a series of normal edits to simulate that it is a regular or novice editor, and then goes into a sleeper account until needed. The account is reactivated when the sockpuppet needs it to reinforce opinion in a discussion, rig polls, or any other unallowed use of alternate accounts.
When a dormant account is suddenly awakened and engages in a very different editing pattern than before, it is possible that this account has been compromised and is in the possession of someone else with malicious intent. For a puppet, taking over the dormant account of someone who left the project allows them to access an account with greater prestige and trust with the community than a new account. Account theft can be forced ( hacking ) or voluntary, in case the original owner is enticed by the puppet to give him access to data.
An account would become a sleeper if the owner does not use it to edit. The reasons may be personal and individual in each case. It could mean a loss of interest in editing Wikipedia, a lack of time, or simply not feeling a desire to edit for a long time. One may edit seasonally. Others take long breaks.
An account may permanently cease to edit if the user has died or has decided to never edit Wikipedia again. There may be no way of knowing this since Wikipedia's identity has not been revealed. It is also possible for the user who left the account to start a new one. This can occur if the user wishes to have a clean start, has forgotten their password, or has forgotten the name of the account.
An account can be created for the sole purpose of sockpuppetry—the unallowed use of multiple accounts at the same time. A sock puppet may perform several "good" edits to suggest it is a normal user, become inactive for some time, then edit later.