How to Find Out What Last Woke Your Computer

Sometimes I let my desktop do some task during the night, and it can be annoying when it wakes up for no reason – lighting up my room with the light from my 34-inch monitor. room to find out that your computer is on, although it shouldn’t, don’t just shrug. With a little detective work, you can figure out what keeps your computer from sleeping peacefully.

What woke my Windows computer?

If your computer wakes up regularly, it can be tricky to find the culprit, but finding the very last reason to wake up is a good place to start. To do this, click the “Start” button and type “Command Prompt”. Right click on the command line listing and open it as administrator. Then enter:

 powercfg -lastwake

The output will tell you what woke your computer up last, which if you didn’t initiate it yourself is likely your cause. (You can also try powercfg -waketimers , which might help.)

If you need more information, you can deepen your search by turning to Windows Event Viewer. Bring it up by typing Event Viewer after clicking the Start button. When it boots up, click on Windows Logs in the left-most folder structure and select System. Next, you’ll want to look at the logs to find the approximate time your system woke up and see what Window has to tell you.

In the example I just ran, the event viewer was able to correctly note that I turned my system back on by fiddling with a USB keyboard / mouse:

Other information you can glean from Event Viewer is whether a scheduled task slept (or woke up) your system, or some network event is to blame (for example, something was using your system’s Wake-on-LAN feature) … You will also see if your system’s power button on the PC turns on, but you would probably remember that (unless you are an avid sleepwalker).

Browsing the source for “Kernel-Power” might also shed a little extra information on why your system is turned on (or off), but it probably won’t be exactly what you’re looking for. I would first stop at checking (or filtering) the Power-Troubleshooter source.

What woke my Mac up?

For Mac : If you want to know what woke your Mac at a specific time, open Terminal and enter the following:

log show —style syslog | fgrep “Wake reason"

It may take a little time to process this command, but when it does, you should see what is throwing your Mac back from blissful sleep. If you don’t see anything, you can also try opening your console (just type this into Spotlight) and search for “reason for waking up.”

If you’re not sure what any of the reasons for waking up means, check out this old glossary from CNET for more information.

This article was first published 03/25/14. It was updated on 10/11/19 with more recent information.


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