Yes I had that issue too when I upgraded to 8.1. My issue was that the network adapter kept waking pc.
Do this to see the devices capable of waking pc:
http://www.sevenforums.com/tutorials/122019-devices-see-devices-able-wake-up-computer.html
If your network adapter is listed, try this:
- Windows key + S
- Type device manager
- Go to your network adapters and choose the one that came up in the cmd prompt
- Right click and properties
- Power management tab
- Uncheck "allow this device to wake the computer"
- Now see if it fixed
If this doesn't fix it, repeat the above steps for the other devices that appear from the cmd prompt.
Hope this helps