Windows 8 has a new feature for its tablets (and some PCs) called Connected Standby designed to give you the "always on" feeling of iOS and Android. It's handy, but it can also be a drain on your battery.
Photo by Karlis Dambrans
Connected Standby is different than "hibernate" or "sleep" modes because it's constantly waking itself up to check for new emails and notifications. For machines with Connected Standby, sleep and hibernate are not available, but you also don't have to wait for the system to boot up. Unfortunately, you can't disable Connected Standby. The How-To Geek explains the options you do have:
While you can't disable Connected Standby, you can get around this limitation by powering the tablet or laptop off if you're not going to use it for a while. The device won't wake up if it's powered off completely. This means going through a normal Shut Down process, not just tapping its power button. You could also enable Airplane Mode before putting your PC to sleep. Your device won't be able to fetch new content or communicate with the Internet at all. It should stay asleep instead of waking up regularly to check on your emails and tweets.
Until Microsoft offers you the ability to disable Connected Standby, it seems you have limited options if you're concerned with saving battery life. But at least you can do something.