When beeing logged in for using the the admin interface and opening the article editor, blogless starts a session. While beeing logged in each time a webpage is loaded the session gets updated. For security reasons you get automatically logged out after not using the admin interface for a specific time. This even can happen while editing an article for a long time without intermediated saving.
Blogless provides a "draft" checkbox in it's editor. This is checked by default, so you can save an article any time and the article gets saved without beeing published yet. And that is what you should do: save often!