How do I stop my MacBook Pro from hibernating?

I purchase my first MacBook Pro (late 2011 model) a few months ago. I Installed OS X Mountain Lion and everything was working fine. I could close the lid, and open it again, perhaps half a day later, and it would wake from sleep and allow me to enter my password.

Now, in the past month (perhaps due to software updates), the behaviour has changed. Now, it goes in to hibernation when the lid is shut for more than a few hours. Upon waking from hibernation it takes a good 30 seconds or more before the clock updates and I can enter my password.

I have been in to System Preferences -> Energy Saver, and changed the settings to never enter display sleep or computer sleep, and I have disabled putting hard disks to sleep when possible. I have also used the pmset utility to set the timeout for hibernation to start to 2 days. I have tried reseting the PRAM and the SMC.

None of this has worked.

Has anyone else experienced this, or does anyone know a way I can stop my MacBook Pro from hibernating so often?

Just to clarify: Everything mentioned above has been when it is plugged in to the wall charger.