Wimoweh has great Plex support, it can stay awake when playing media then sleep afterwards.  It also works if Plex is on a different machine

Wimoweh supports Java applications and can keep the system awake automatically when they run

Wimoweh will automatically keep your Mac awake whenever a particular application or process is running

Wimoweh can also allow you to keep your Mac awake based on activity on your local network.

Keep your system awake based on how busy it is – set the CPU usage threshold and a timescale

Stay awake forever or use a countdown between 1 minute to 12 hours

Stay awake on a schedule, e.g, never sleep between 9 and 5 on weekdays

Instant visibility of all power demands on the system

Stay awake automatically when using VPN – no more coming back to a dropped connection

Match processes using a partial string – great if you spawn multiple processes and want to match them all

“This is one of those really useful apps with features that Apple should take notice of, and incorporate in OS X”

Mac App Store Review (UK)

”It is by far the best power managing third-party app that lays on top of OS X’s already amazing power management system.”

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Got a question, crash, or request? Get in touch with the developer, he goes above and beyond to get it right.

Mac App Store Review (USA)

Easy to use and does exactly what it says. Now I’m able to keep select apps (like dvdfab) running without risk of OSX sleeping. The color coding to show its active is a brillant idea. This program is a dream come true.

Mac App Store Review (USA)

This app gives you better control over how your Mac sleeps. Simply the best in business, worth the money!

Mac App Store (Brazil)

"This is by far the best power manager tool I have ever used on Mac. I have tried so many, also the very common ones like Caffeine but this one really gives you all the functionality you need... In addition, the support of the Wimoweh guys is amazing! Sent them an email about a feature request and I got an immediate response and they will just implement it. I can recommend this tool to everyone looking for an advanced power manager for the Mac, to overcome the limitations on the Mac."

App Store Review (Switzerland)