How to identify processes that prevent a mac from sleeping

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Run the following Terminal command

pmset -g assertions

The 1st section of output shows the status of kernel assertions. If either PreventSystemSleep or PreventUserIdleSystemSleep have a status of 1 then the mac will not sleep.

In the 2nd section of output the processes that own the assertions are shown(process id then bundle id).

To identify the process:

ps up <pid>