Sleep mode is a default account preference that affects how Fleep works on the desktop version (web browser, Mac, Windows and Linux native apps). The main idea of Sleep mode is to prevent new incoming messages being marked as read, when you are actually away from your computer or otherwise not paying attention to your screen, yet have a conversation open.
To prevent such cases where new messages could get marked as read automatically, we have introduced Sleep mode. Effectively, it means that if you leave Fleep open, active, on your screen but you are inactive on your computer (that is, not doing anything actively in Fleep) then Fleep ‘freezes’ your current open conversation, so new incoming messages on the screen are not marked as read.
You can find the Sleep mode setting in your Account preferences:
You can always configure the default Sleep mode setting for a longer 'timer' or even disable Sleep mode altogether, if you'd like.