iMac Display Won't Wake From Sleep

50% of time, imac display won't wake sleep. can hear computer still on, display won't turn on. way fix has been restart computer
if change energy saver settings computer never sleeps works, not wanna leave on 24/7.

issue tends happen after computer has been asleep longer period of time (about 7 hours).

how fix this?

 

about how long trying wake before giving up? when it's sleeping, how do restart?

 

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