The simple answer is "nothing".

There is a more complex answer though. Longer hours on does reduce the service life of certain components. Measurably? Probably not. It will also cast extra heat in your room- though that may not be a concern. It will waste electricity if there is 8 hours of extra idling compared to before- but again, it's hard to say how large of an impact this will be given your hardware. It could cause burn in on cheap crappy monitors too- but again this is pretty unlikely.

Now- all that said- why would you want to? Is it because your PC takes too long to start up? If that is the case you could probably recoup the cost of buying an inexpensive SSD for your operating system to lower your boot time within a relatively small amount of time.

8 extra hours of idling for truly nothing may not cause extra harm to your components but why?