How can I prevent my computer from waking up accidentally?

If you type in your elevated command prompt powercfg /waketimers, it will display a list of scheduled tasks that will wake the computer. You might discover what could be causing this to occur.

If it is a wake timer that's causing this behavior, if you go into the Advanced power settings, under the Sleep category, you can disable Wake Timers. This might fix your issue, if this is the culprit.

For instance, I see this when executing that command:

EDIT:

Also, by the way, I did recall this occurring with my laptop once. It took a bit of troubleshooting, but I discovered that the culprit was my Microsoft Arc Touch Mouse. I discovered that the USB dongle (which received its wireless signal), caused the computer to periodically wake up, even though the mouse wasn't powered on. I fixed the issue by finding the USB device in the Device Manager and, under Properties, turning off "Allow this device to wake this computer". However, I found a more prudent solution was to just not leave it plugged into the laptop all the time. :)

I've had this problem before when I had network shares that were hosted on the sleeping machine, but accessed by other machines on the network. There are some settings for network cards that determine if it can wake the machine from sleep, and under what conditions:

This TechNet article explains the power management settings for network devices on Win7. There's not a Win8 version of this page yet, but I assume they are somewhat similar.

Beyond this, many other devices with similar checkboxes to allow for waking - I'd probably advise turning as many of these off as possible, and seeing if your problem persists. If it does, you might consider a binary search style approach to figure out which one is the offending sleep-disturber.

You should check if you have anything similar to this in your BIOS:

Setting everything to disabled at the "Power" tab should override any Windows settings.

I fixed this by doing the following:

1. Go Device Manager
2. Selecting each keyboard and mouse device and right-click Properties
3. Go to the Power Management tab and untick "Allow this device to wake the computer".

There might be a scheduled task for the night that wakes up the computer.

I would check "Schedule tasks" (or whatever it is called on Windows 8), and specifically in your case the task of Microsoft / Windows / Time Synchronization / Conditions tab / check-box "Wake the computer to run this task".

You can also disallow all wake-up timers in Power Options / Advanced settings / Sleep / Allow wake timers.

Some helpful articles (and their comments) :

How to Fix a Computer that Keeps Waking Up
How To Prevent Your Computer From Waking Up Accidentally