I don't know what will happen when in Windows logon screen
but you can change the settings in Power Setting
you can set how long the Screen idle before it turn off (just the screen) or how long the laptop idle before it will go to sleep
as Philip indicated once you launch the recovery mode, the system uses internal software that basically opens a compressed file and slaps it on your hard drive destroying everything underneath it, so back up you data before you do this. most reimagine take a few minutes, but your fine it will not go to sleep during the process as it will be suspended anyways.