Have a look at this screenshot, captured with the snipping tool, in Windows 8 Release Preview.
The task manager shows that my system is up for 1 day and 23 hours and 41 minutes (thats 19 minutes short for the second day). But actually that isn’t the case. I turned on my PC just minutes before this screenshot was taken and after a proper Shut Down (Charms -> Power -> Shutdown).
Want to try out on your own? Just press CTRL + Shift + Esc and go to Performance tab. You can also do some more. Keep your power supply on (in case if you use a PC) shut down your system and hit the spacebar twice or thrice. Your computer will turn on automatically. There is even more. Open Notepad. Type something. Shut down and turn on your computer. And you could find the notepad ready for you.
So why does it happen?
The answer is “Windows 8 never actually shuts down”. Yes. Never indeed. In whatever way you try to shutdown your PC (even by executing shutdown.exe), the computer goes to a hibernate-kind of mode. This is not actually explicitly mentioned as a feature of Windows 8 by Microsoft. But they do have a name for this. They call this procedure “Fast Startup”
Fairly speaking, there is nothing to worry about this feature of Windows 8. It neither consumes any power nor does it change the aspects of computing. It just makes our lives pretty easy. But the question is why doesn’t Microsoft publically announce this as a feature of Windows 8? Is there something unannounced hiding inside Windows 8? Well, answers will be on October 26.