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 amount of time free of an edit makes an account a sleeper. This is just an informal term for an account that appears it may never be used again. Many editors do take long unannounced breaks from editing, edit only on rare occasions, or get back into Wikipedia after a long absence that could be months or even years.

Provided the account is in good standing, and the same person is not operating another account for which operating both would violate sock puppetry guidelines, there is no objection to resuming editing after a long absence.

An account would be a sleeper if the owner simply does not use it to perform any editing. The reason is very personal and individual to each case. It could mean loss of interest in Wikipedia editing, a lack of time, or simply seeing no need to make any edits for a long time, among others. Or one may edit seasonally.

An account may permanently cease to make edits if the user has died or has decided never to edit Wikipedia again, though there may be no way of knowing this, since Wikipedia is anonymous.

It is also possible that the user abandoned the account in favor of starting a new one. This could occur if the user wishes to have a clean start, has forgotten their password, or forgot even the name of the account.

An account can be created for the sole purpose of using it as a sock puppet (the unallowed 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, and then stop editing for some time, to awake in the future on the right time.

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.

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 use it in the same ways you always did.

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 up to 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.