Windows Power Management.
Laptops are tricky as there are 2 settings, when: 1)Plugged in, 2) on Battery
I will
assume 
you are in Battery mode as the time is short.
Normally any interaction (mouse or Keyboard) resets the timeout.
How slow are you reading ?

Auto scroll, no user action ? Right?
Maybe Kovid can add a Keep alive OPTION like vid players have