Anxiety can trigger ad infinitum ruminating that makes our brain scan for potential problems. As you lie down to sleep, your mind just won't stop thinking about what may go wrong.
Ruminating easily takes over your brain when you are doing nothing, and your mind is idle. That is crucifying.
Although ruminating is common among people with depression and mental disorder, it is extremely common among healthy people, too. However, they don't experience ruminating as frequent as those with depression.