Old 2010 iMac reboots at night, every night. Why?


This is my old 2010 27in Imac. Used as a main machine for about six years, then demoted to 2nd spot to run things like the CCTV system and stuff, now to third, which is for very occasional use, mostly when I need to use an app that won’t work on newer OS’s.

When this mac was in first and second place it was on 24/4. Never slept, just the screen turned off. It did this as it was always running background processes (as I say, CCTV, backups, etc.).

Since being demoted to third spot it’s spent most the last few months turned off. But this week I’ve had the need for it to go back to being on 24/7 to run some background stuff (email list processing).

Power setting are correct: Sleep: Never. Powernap: No. Etc.

The first day it was “always on” it had rebooted over night. I put this down to a one-off, but it’s happened every night this week. When I last check it at night it’s running fine, but morning the apps are no longer running and the machine has clearly rebooted itself. This only happens over night, never during the day, ruling out an over-heating issue, I think.

I thought maybe one of the apps that it’s running is perhaps causing it, maybe it won’t run more than say 12hrs. So yesterday I didn’t start the apps until last night. But again, this morning it’s rebooted.

Anyone any ideas?

I don’t think this is happening due to a failure, as it’s always happening overnight, at what seems like a fairly regular time (somewhere between midnight and 6am.

It reminds you it’s still there and doesn’t want to be replaced by a Mac mini sooner or later…


It was replaced by an M1 MM a few weeks ago.

I’ve a couple of really old apps, mostly for email list management, that will only run on older OS’s, so I keep it hanging around for that purpose. Plus, I’m curious to see just how many years service I can get out of it ;-)

Kernel panic - your iMac will go through some basic automatic system clean up operations at around 3am (defragging recent files etc). It’s probably hit a problem during this process.

Resetting your parameter ram sometimes clears any underlying issues by forcing your iMac to reset basic system setting (time, location etc).

Shut down your Mac, then turn it on and immediately press and hold these four keys together: Option, Command, P, and R. Wait until your hear two start-up chimes and release the keys allowing your iMac to start up.

After this, check your System Prefs for power saving and sleep options.


Ohh, didn’t know the 3am thing. Many thanks Geoff.

Of course, I now have this song in my head, and there it’ll remain for the whole day i suspect!

The legend that is Bill Drummond, The KLF, and the song 3AM Eternal.