Black screen issue - daily!

I have a 27inch late 2012 iMac that seems to be having a black screen issue.

At the end of the day, my iMac goes into sleep mode. In the morning, I come into my office and find that there is a black screen, but with some light coming from it. i.e. it is not black as in switched off, but dark if that makes sense. Nothing I do affects it. This has been happening more and more and now it happens on about 90% of the days. I have to hold in the power button to turn it off. Then I turn it back on and it starts booting. It makes the usual mac noise on power up, but then a few seconds later, it makes the same noise again!

Then, the boot up completes and all is good. That is until the next day!

I have turned Schedule off so it is not supposed to start up in the morning anyway, but whenever I come in, there it is, dark screen with some light from it.

Any idea what is going on?