Same thing happens to me. I'll put my iMac to sleep and return a few hours later to find it off. After turning it back on, I'm always greeted by a lovely "Sleep/Wake Crashed" message. But it seems to be resolved with the new 10.11.6 BetaSometimes after not using my 5K for a day or two I come back to it and it seems to have turned it self off. Is this a common problem? Any ideas of what might be wrong?