I get the doomsday dreams when experiencing low blood sugar. Not really dreaming, but zonked out with the low. Then when I recover from the low blood sugar my mood stabilizes and I can't believe I felt so doomed.
I take one diphenhydramine tablet (generic Benedryl) every night, 2 hours before I go to bed. Walgreens has Waldryl, CVS has Allergy tablets. I stock up when they are on sale. By the time I go to bed I am very sleepy, and usually manage to sleep through the night unless I need a potty break.