Just recently I have had a problem with my iMac that when I wake it from sleep mode the screen is a textured grey colour with all the desktop icons showing faintly. There is no mouse pointer. If I press the power button briefly the screen comes up as normal.
The grey screen with faint icons is a sign of "deep sleep". A short touch on the Mac's start button should liven it up nicely -- a long press will cause a restart.
Safe mode boot followed by a 'normal' reboot is useful and may help to stop this slow wake issue (if it recurs) by clearing caches which could be responsible, along with other housework.
Lionel Ogden 