Glossary
Sleep assertion
A request from an app to macOS to keep the Mac or its display awake.
What it is
A sleep assertion is a request an app makes to macOS to keep the Mac, or just its display, awake. Video calls, downloads, backups and media playback use them so they aren't interrupted. If your Mac won't sleep or the battery drains overnight, an assertion an app forgot to release is the first thing to check.
The common ones are PreventUserIdleSystemSleep (keep the Mac awake), PreventUserIdleDisplaySleep (keep the screen on) and PreventSystemSleep. Most are short and end on their own. A Mac that does sleep but keeps waking up is a different problem: see dark wake.
Why it matters
If your Mac won't sleep, or drains overnight, an assertion held by an app is the first thing to check.
In Activity Monitor
The Preventing Sleep column in the Energy tab shows which apps are preventing sleep right now.
In iKnowMyMac
Sleep & Battery lists every app keeping your Mac awake now, the kind of assertion and for how long, and keeps that history for the overnight report.
pmset -g assertionsLists assertions system-wide and by owning process.
FAQ
Questions
Something else? support@iknowmymac.com
What is a sleep assertion on a Mac?
A request from an app or system process asking macOS not to sleep, or not to turn the display off, until it finishes. pmset -g assertions lists every assertion and the process that holds it.
How do I see what is keeping my Mac awake?
Open Activity Monitor's Energy tab and look at the Preventing Sleep column, or run pmset -g assertions in Terminal and read the lines under “Listed by owning process”. Each line names the app, how long it has held the assertion and its type.
What does PreventUserIdleSystemSleep mean?
An app has asked macOS not to put the Mac to sleep when you stop using it. The display can still turn off. PreventUserIdleDisplaySleep keeps the screen on as well.
Can an app wake my Mac without a sleep assertion?
Yes. An assertion keeps the Mac awake; a scheduled wake request wakes it later. Those appear as Wake Requests in pmset -g log; Mac keeps waking up from sleep shows how to read them. Energy impact won't show either while the lid is closed.
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