iMac won't wake from sleep following Yosemite upgrade

Hi, I took the plunge and upgraded my iMac (21 inch) to Yosemite on Monday and now I really wish I hadn't bothered! Up until then I had had no problems with it but now since there have been a number of occasions when I can not get it to wake up from sleep. The screen is black and no matter what I do it will not wake up. In the end I have no choice but to force it to quit by holding down the power button and restart it.

It's very frustrating, especially because I work a lot in Lightroom and when I force quit it is closing my LR catalog without backing it up.

I have found a few threads from the end of last year where people have been experiencing similar problems but I haven't found any with a solution to it. I wonder if anyone might be able to help......

Many thanks Jude