You are interested in how is sleep affected in people with brain injuries? (Possible angles include: is sleep affected differently in different populations; or does it depend on the type of brain injury;...or...or...).
While Googling you see this article, and it peaks your interest. What now?
Step 1: Break up this research question into two or three main concepts. Ignore extraneous words.
Step 2: Conduct an initial search with these words, and quickly scan the results.
Step 3: Using information from your initial search, add synonyms, acronyms, and variant spellings.
(sleep OR insomnia OR hypersomnolence) AND (brain injury OR tbi OR concussion)