If your Mac won’t go to sleep, try this:

pmset -g assertions

Check the output for details on which processes and voodo magic is keeping your iMac up all night:

Listed by owning process:
   pid 4627(cloudd)...
Idle sleep preventers: IODisplayWrangler

Things you can try:

• Resetting the printing system (CTRL+click in the left pane of the Printers and scanners settings)
• Unplugging external devices (USB, bluetooth, etc)
• Voodoo

References:

https://discussions.apple.com/thread/7256162

https://www.reddit.com/r/applehelp/comments/43ks7u/macbook_wont_go_to_sleep_hidd_preventing_sleep/

https://apple.stackexchange.com/questions/307151/significance-of-iodisplaywrangler-in-power-management-assertions