How to Figure Out What’s Preventing Your Mac From Sleeping

It’s bad enough that your kid never goes to sleep, but now your Mac is doing the same thing! Sure, it may not be crying, but your Mac just sits there, awake, without giving you any indication as to why. What’s going on?

It could be anything. A file transfer or backup may be ongoing, or you might have prevented your Mac from sleeping only to forget later on. The only real way to find out is the open the Terminal, which you can find by using Spotlight or by heading to Applications Utilities in the Finder.

From the Terminal prompt, run the following command:

pmset -g assertions

Then hit Enter. The output comes in two parts. First, you’ll see a list of categories followed by a number.

These are all kinds of things that can keep your Mac awake. A zero next to the category means it’s not currently active; a one means it is.

What these mean is largely self-explanatory. “BackgroundTask,” for example, means that your Mac is doing something like backing up to Time Machine or downloading an update—this prevents it from going to sleep. “UserIsActive” is pretty much always going to be active, because you just used the computer to type the command.

Below this relatively straightforward list is some more convoluted language. For example, if Time Machine is running, you’ll see something like this:

It’s a bit cryptic, but let’s break it down. First off, pid strands for Process ID, and it’s followed by a number representing the process. You can use this number to identify the process in Activity Monitor.

In brackets following the process ID number, you’ll see the name of the application in brackets. In the above example, this includes backupd and UserEventAgent, both processes related to Time Machine. This is followed by further identifying information, then a timestamp, then a bit more information regarding why the process is keeping your Mac awake. As you can see, a running Time Machine backup is relatively easy to identify.

But there are many other reasons your Mac might be staying awake.

If you have music playing, you’ll see something related to coreaudio preventing sleep. If you have a USB or Bluetooth device connected, you might see a note about those preventing sleep. And if you’re running an app like Amphetamine, which keeps your Mac from sleeping, you’ll see a note about that:

There’s no end to the potential causes of a Mac that won’t fall asleep, but running this command at least gives you a list of potential things to consider. If you recognize the cause of your Mac staying up, you’ll need to either shut down the process causing the problem or change its settings.

If you don’t recognize something in the list, Google it. That may not seem like the most helpful advice, but there are simply too many possibly problems for us to list them all. Happily, the Internet is full of people just like you having problems, and in most cases some other very nice person has found a solution for them. Seek them out, and good luck!