Ever since I upgraded to the latest El Capitan I have noticed that after my computer goes to sleep with the external display, the iMac will wake up but the external monitor will remain sleeping. The iMac will go to the login screen but then flash black and back as if it's detecting a display over and over. The only quick fix is to turn the external off and on again. I just want my seamless sleep cycle again damn you El Capitan!!! Has anyone else experienced this?? I've seen a few posts about sleep cycle issues on here earlier but none specifically about the external display issue I'm having.