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A sleeper account is an account that has not edited in a long time and has essentially lain dormant. Technically, a sleeper account is still active and it is still possible to use it. However, any such use of an account must conform to Wikipedia guidelines, particularly those of sock puppetry.
No specific period without editing makes an account a sleeper. "Sleeper" is an informal term for an account that currently appears to be dormant. Many editors take long, unannounced breaks from editing, edit rarely, or s/he 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.
Why would an account become a sleeper?
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 loss of interest in editing Wikipedia, a lack of time, or simply not needing edits for a long time. One may edit seasonally.
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 is anonymous. It is also possible the user abandoned the account to start a new one. This can occur if the user wishes to have a clean start, has forgotten his or her password, or has forgotten the name of the account.
An account can be created for the sole purpose of sock puppetry—the disallowed use of multiple accounts at the same time. A sock puppet may perform a number of "good" edits to suggest it is a normal user, become dormant for some time, then awaken later.
Treatment of a sleeper account
As long as a sleeper account is in good standing, and all account usage guidelines are followed, the user has the right to the account to resume using the account any day, even after years of absence. The user retains the right to the user name indefinitely.
Communicating with the owner of a sleeper account may be impossible or difficult if the owner leaves no other contact information behind.
It is possible that even in the absence of any edits, the owner of a sleeper account is logging in frequently and checking his/her watchlist and talk page for any activity. But there is no way of knowing.
Revival of a sleeper account
If you are the owner of a sleeper account, you are welcome to revive it at any time, provided you follow Wikipedia guidelines pertaining to multiple accounts. Once you do so, you may resume using the account like normal.
You may find that some articles you created or edited have been deleted when you weren't looking. If you want to know why, see Why was the page I created deleted?
You may find quite a lot has changed. A watchlist can only show the most recent 30 days of edits. But you can examine the individual edits of a page if you wish.
You may find some messages that one or more editors have left for you, and they may pertain to deletion or something else. If the message is about an article that was proposed for deletion, you may need to ask an administrator to retrieve the article and put it into your userspace, where it can be improved with an effort to put it back in the encyclopedia.
There is always the possibility that a sleeper account could be taken over by a malevolent person, who might then use the account for unacceptable purposes such as sock puppetry.