If the game itself has a sleep mode then yes. If you're using PocketNES to play NES games then that too can be put into sleep mode, I have no idea if NESDS does that too. Otherwise, I don't think you can put them to sleep. This also works on the GB/GBC for GBA emulator Goomba/Goomba Color.
you can patch the gba games with gbata to incorporate sleep mode. L+R+Start/Select to come in/out of sleep mode, i forgot which one is which by default.
Awesome! Thanks a million. I searched for the software on Google and found the program. And it works! You can also set your own button combination for sleepmode and stuff.