Mouse cursor won't move on waking from sleep in iMac

I have a late 2014 27" iMac that has always worked like a champ. Here recently however, when waking up from a sleep, the mouse curser would not move, I was dead in the water.

At first I thought it was just a battery that needed changing in my Bluetooth mouse, but to no avail. I swapped out the mouse for another one and put in fresh batteries, but again it did not work.

I checked the batteries with a tester and they were all up to full strength.

I then powered down and restarted my computer, now the mouse was working perfectly.

This has now happened on four or five separate occasions in the last two months and it always happens from a wake-up. As soon as I reboot the computer everything is fine again.

Does anyone have an idea as to what the problem is? It is unlikely that the problem is with the mouse since replacing the mouse with another doesn't work. So, it must be a computer, or software issue; but why?

If anyone has a clue as to what the problem is, I would be very interested to hear your opinion.

[Re-Titled by Moderator]