Some might notice since they had installed Windows 10 on their PC, Sleep Mode and hibernate are missing in Power section or area.
Some had concluded already that it depends on PC, but it is not.
Following this guide can help you fix or enable it on your PC.
Click Start, type: Power Options
Under Control Panel Home, click Choose what the power buttons do
Click the link Change settings that are currently unavailable
Then scroll down to the bottom of the window and ensure Sleep is checked and click Save Changes
You should now see them listed when you click Start > Power