I think Hibernate option doesn't come by default with Windows 8, and we will have to enable it manually. This is the same question everyone new Windows 8 user has, I was one of them and so including this steps for everyone who will be and who want to use Windows 8 with Hibernate option enabled.

1. Go to Power Options page as shown below. 


There are different ways to go to that page, simple method is from Windows 8 StartBar > Click on Battery icon and select More Power options as shown in below screenshot.

2. Select any one of the options except "Require a password on wakeup" present on the left side of the bar of Power Options page.

I have selected 2nd option - "Choose what the power buttons do"

You will see similar page below.. where you can see at the bottom of the page we have Hibernate option available but not selected.

3. Enable Hibernate option, first click on "Change settings that are currently unavailable" which will make that option editable so you can Enable that checkbox.

4. Save changes and now you will see new Option Added to your Power Menu.