Sharing Files between 2 computers on a network if one is in sleep mode

My computers, one downstairs and one upstairs, see the other computer in the home group, when they are both on and I can access files between them. Is there a way to access the other computer depending on whether we are upstairs or downstairs when the other computer is not on and is in standby or sleep mode.

Not as far as I know, both computers must be turned on to be used, as being in sleep mode means that the computer don't respond to any commands other than to wake it.
An alternative would be to setup and use WakeOnLan feature, so that you can turn on computers remotely.