Have you done this -
1) In the start search bar type Power Options and hit enter.
2) On the left you should see some options "Choose what the (name) button does".
3) If you're using a laptop you should have some options such as "Shutdown Button", "Sleep Button", "On lid close", etc.
4) Most likley your Sleep Button setting is set to "Do Nothing". Make the desired changes and exit.
If you have already done that it could be your mouse -
Mouse: Easy solution . Do this:
Control Panel -> Device Manager -> Mouse -> Right-click your specific mouse -> Properties -> Power Management tab -> uncheck "Allow this device to wake the computer"