Probably because some process or peripheral such as your mouse and keyboard are by default, set to wake your computer, and you haven't yet disabled that ability.
There are several places you have to go and check, to ensure this doesn't happen.
#1
I always start with attached devices that can wake a machine, and disable those devices that should not be waking the machine. To perform this, do the following:
Left mouse-click the Start menu and type
command. In the
Best match list that is presented, left mouse-click
Command Prompt.
Type
powercfg /devicequery wake_armed into the command prompt and press
Enter, and you'll be presented with a list of devices that can wake the computer from sleep. Typically you will see mice, keyboard, and networking devices. Having peripherals set to wake the computer often results in seemingly random wake up events, as devices such as a computer mouse tend to "see" even slight movement from time to time unless turned off or flipped upside down so the sensor is essentially blind.
You can disable a devices ability to wake the computer through Device Manager.
Right mouse-click the Start menu and choose Device Manager to open it.
In Device Manager, look for devices that were listed in the command prompt under their corresponding categories. Double left mouse-click or left mouse-click the small triangle next to the device category you wish to expand, to find the device you want to disallow to wake the computer.
Double-click the device to view it's properties and select the
Power Management tab. On the Power Management tab, uncheck
Allow this device to wake the computer.
Click
OK to save the changes.
Do this for each device listed in command prompt that shouldn't be waking the computer. You may have to look at several devices before you find the correct one that has the Power Management tab in it's properties. The names aren't always fully descriptive, and you may see what appear to be duplicates of hardware devices.
After disabling wake for your hardware devices, you can rerun the powercfg query at the command prompt to verify that those devices are no longer listed as able to wake the computer.
#2
Left mouse-click the Start menu and type
command. In the
Best match list that is presented, right mouse-click
Command Prompt and choose
Run as administrator.
Type
powercfg /waketimers into the command prompt and press
Enter, and you'll be presented with a list of software tasks that are scheduled to wake the computer from sleep.
You can modify scheduled tasks ability to wake the computer through the Task Scheduler.
Press
Windows Key+R, type
taskschd.msc, and click
OK, to open it.
Browse through the
Task Scheduler Library group by double left mouse-clicking it, and double-clicking subsequent groups until you have located the task you want to disallow waking the computer for.
Double-click the task to view it's properties and select the
Conditions tab. On the Conditions tab, uncheck
Wake the computer to run this task.
Click
OK to save the changes.
Do this for each task listed in command prompt that shouldn't be waking the computer. You may have to look at several scheduled tasks before you find the correct one that has
Wake enabled on the Conditions tab in it's properties.
There are some tasks that Microsoft will revert back to waking the machine for, so this is not always a complete solution. If you find a task that is being reset by Windows to wake the machine, you will need to take ownership of the task and set it to read only, which should effectively prevent Windows from modifying it at that point. I'm not including instructions for that here as the post is already long enough.
#3
Disable Wake Timers for each power profile through the Power Options for Windows.
Press
Windows Key+R, type
control powercfg.cpl, and click
OK, to open it.
In the Power Options window, click
Change plan settings next to one of the listed power plans. You may see one or more plans. On the Edit Plan Settings window that opens, click
Change advanced power settings.
In the Advanced settings window, locate the
Sleep group heading and expand it to find
Allow wake timers.
Set
Allow wake timers to
Disable and click
Apply to save your changes. Perform this change for each power plan by selecting a different power plan from the drop down box and performing the same steps to disallow wake timers,
Applying the changes for each power plan to save them.
Hopefully this helps you track down and prevent your computer from randomly waking from sleep.
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