The bonus affects skillgain and stamina regeneration.
You can check the bonus with the new /sleep command.
==060913==
* Fixed a bug where you could not rent a bed unless you were holding an item.
* If you sleep less then 3 hours real time, your bed will still be available to you if you hired it.
* You now receive a question before going to sleep.
* Bug with setting sizes fixed. The sizes of cotton rags were also made smaller. Combine old rags to get the new size.
==060912==