To resolve Windows 8.1 sleep issue try below workarounds,
1. Check Power Management OptionsFirst step is to check your power options & make sure Sleep option is selected.
Open power options from Control Panel. Click on Change plan Settings.
Click on Change advanced power settings.
Under Power buttons and lid and then click the + and verify the action is really set to Sleep as shown below and Apply the settings.
2. Disable IPv6 optionsBy default all network adapters have both IPv4 and IPv6 versions enabled but you can disable IPv6 as shown below.
Go to Control Panel and open Network and Sharing Center. Click on Change adapter settings as shown below.
Right click your Ethernet adapter and click Properties as shown below,
Uncheck Internet Protocol Version 6 (TCP/IPv6) option as shown below and apply the settings.