Try moving from general to more scholarly:
Sleeping problems to insomnia
Mood issues to depression
Try moving from broad to more specific
Sleep supplements to melatonin
Start out with 2-4 keywords and review your hits
Ex. insomnia, depression, melatonin
Look at other keywords/subjects listed with your results
Ex. circadian rhythms
Want help brainstorming keywords? Try:
How to Generate Keywords
http://www.lib.utexas.edu/keywords/index.php
Source: University of Texas at Austin (CC-BY-NC 3.0)
OR
Search Strategy Generator
http://www.lib.umich.edu/search-generator
Source: University of Michigan Library