Sometimes and even in the middle of the night, my iMac will wake, with no one around and come up in a bright blue and bright pink login screen. The computer is frozen for a few seconds to almost a minute when in this state then the regular login screen will appear.
Is this a glitch, a faulty CPU or screen or what? Is there a fix?
iMac Line (2012 and Later)
You can try some basic resets such as an
NVRAM reset https://support.apple.com/en-us/HT204063
Restart in Safe Mode https://support.apple.com/en-us/HT201262
However if it neither of these it may be a hardware issue such as a failing GPU. Please do the resets first and if those do not help, schedule a trip to the Apple Store (assuming the iMac is a 2013 or later) or to an independent AASP.
You can try some basic resets such as an
NVRAM reset https://support.apple.com/en-us/HT204063
Restart in Safe Mode https://support.apple.com/en-us/HT201262
However if it neither of these it may be a hardware issue such as a failing GPU. Please do the resets first and if those do not help, schedule a trip to the Apple Store (assuming the iMac is a 2013 or later) or to an independent AASP.
In addition to rkaufmann87's suggestions I would do the following if the issue continues:
Try creating a new user. If the issue goes away then there may be some process or app that is waking the system to perform a task. Evaluating the log files and/or Activity Monitor may provide clues. You could also upload an EtreCheck report, which can be generated using the app available at their website, so that others can provide input on your specific iMac configuration and possible solutions (i.e. remove malware, update specific software.
How to use Activity Monitor on your Mac - Apple Support
Another option is to try reinstalling macOS. Make sure you have a backup of your data just in case.
Random blue and pink login screen waking from sleep
