(If you are not a Caffeine user - I suggest, you stop reading and keep looking for alternative solutions ;-) )
This could be due to numerous reasons like the Mac connected to a network drive, an USB connected to it which reads data among others. But, the specific issue I had was with this very useful little utility called Caffeine - it installs a small little icon which when you activate doesn't let your Mac go to sleep if you have a short time set for the screensaver or for power saving. It comes handy especially when you are maybe reading a long text and the computer just blanks out due to the settings, or have to answer a call (any type ;-) ) suddenly, but do not want the computer to go to sleep and type in the password again!
Now, though it helped me a lot of times - it seems to be the cause of an much more irritating issue - the Mac wakes up after every 2-3 hours for seemingly no reason at all! no attached USB drives, no accidental movement of mouse or keyboard, no low power to the either of the input devices. I say irritating as during the night when its all dark and you are trying to sleep, the login prompt suddenly lights up almost the whole room! (maybe my default settings are to a higher brightness level...but just for the login prompt!)
I had tried everything, including taking it to the Apple store - though they tried scanning the system for device, memory issues, nothing popped up and they could only suggest reinstalling the OS as the last resort (one of the many reasons I had moved from Windows ;-) )
And suddenly after an apartment move, I accidentally noticed that if the Caffeine was turned off - the computer would not wake up in the middle of the night!
I now have no proof of what happens and how it does that, but I have successfully reproduced the issue and its "remedy" for quite a few days now and while I look up more on why this happens, if anybody else is facing this issue - try turning off the Caffeine before going to sleep!
This could be due to numerous reasons like the Mac connected to a network drive, an USB connected to it which reads data among others. But, the specific issue I had was with this very useful little utility called Caffeine - it installs a small little icon which when you activate doesn't let your Mac go to sleep if you have a short time set for the screensaver or for power saving. It comes handy especially when you are maybe reading a long text and the computer just blanks out due to the settings, or have to answer a call (any type ;-) ) suddenly, but do not want the computer to go to sleep and type in the password again!
Now, though it helped me a lot of times - it seems to be the cause of an much more irritating issue - the Mac wakes up after every 2-3 hours for seemingly no reason at all! no attached USB drives, no accidental movement of mouse or keyboard, no low power to the either of the input devices. I say irritating as during the night when its all dark and you are trying to sleep, the login prompt suddenly lights up almost the whole room! (maybe my default settings are to a higher brightness level...but just for the login prompt!)
I had tried everything, including taking it to the Apple store - though they tried scanning the system for device, memory issues, nothing popped up and they could only suggest reinstalling the OS as the last resort (one of the many reasons I had moved from Windows ;-) )
And suddenly after an apartment move, I accidentally noticed that if the Caffeine was turned off - the computer would not wake up in the middle of the night!
I now have no proof of what happens and how it does that, but I have successfully reproduced the issue and its "remedy" for quite a few days now and while I look up more on why this happens, if anybody else is facing this issue - try turning off the Caffeine before going to sleep!
