Glossary
Sleep assertion
A request from an app to macOS to keep the Mac or its display awake.
What it is
Apps that shouldn't be interrupted, such as a video call, a download or a video playing, ask macOS not to sleep. That request is an assertion. The common ones are PreventUserIdleSystemSleep (keep the Mac awake), PreventUserIdleDisplaySleep (keep the screen on) and PreventSystemSleep.
Assertions are normal and usually short. A forgotten one can keep a Mac awake all night.
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.
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