Amphetamine - Ultimate Sleep Prevention for Your Mac
Mac Apps
Preventing your Mac from going to sleep is as easy as opening a terminal window and entering the caffeinate command. If you do that, your Mac will stay awake until you type the uncaffeinate command. Easy right? If you want more control, however, I suggest getting the free Mac utility by William Gustafson, Amphetamine for its granular settings. In its most simple iteration Amphetamine keeps your Mac awake until you turn it off, until a file finishes downloading or for a given duration. You can build complex triggers in the settings using criteria like whether a certain drive is plugged in or what the battery level is. There are some interesting extras, like keeping spinning hard drives awake or simulating mouse movements. If you are disturbed by the menu bar icon resembling a pill, there are several to choose from including the classic coffee cup used by the deprecated app Caffeine from Amphetamine is descended.
My personal favorite use case is one I use every two weeks when my house cleaning service comes in. That's the only day I close my MacBook. I use the setting to keep my Mac awake while closed until 5:30 PM. That way if I need to remote into it during the day, it's available without me having to leave it out in the way of the housecleaner.