If you are the owner of a late-2013 Mac Pro, then an update is available that addresses some bugs and enables new Power Nap modes.
The update is a small 544KB SMC firmware update, that will allow the Mac Pro to now run in Power Nap mode without running the fan that is located at the top of the unit. This will make the system run completely silently when in sleep mode, while still performing Power Nap functions like Spotlight indexing, iCloud syncing, receiving new Mail messages, Backup, and downloading software updates from the Mac App Store.
In addition to Power Nap augmentation, this update fixes an issue where low-speed USB devices like mice and keyboards may not be detected by the system at boot times, preventing the passage of boot arguments for entering modes such as Sleep, Single User, and Verbose mode, or getting to the boot menu or Recovery volume.
The update should be available in Apple's Software Update service, which can be accessed in the Apple menu. Apple has also made it available as a standalone download on its Support Downloads page, which can be applied manually, if desired.
Being a firmware update, be sure to follow all on-screen instructions, and expect the system to reboot at least once while installing the update, during which you may see a special progress bar displayed, or even see a black screen show for a few moments. Do not force the system to reboot, unless the system is unresponsive for an extended period of time. While modern systems contain measures to help protect firmware updates, it is possible that an incomplete or otherwise faulty update can break a system and require servicing to fix.
As always, be sure to fully back up your system before applying this or any other system or firmware updates.