Can I split Ambien (zolpidem) in half, 5mg before bed and 5mg in the middle of the night. When I can't sleep anymore?

Yes, but... ...That's not ideal. For some people, ambien's 2.3hr half-life is too short. First i'd try switching to Lunesta (6 hour half-life) or Ambien cr. If those are too long-acting (causing morning "hangover"), i'd go back to Ambien 10 at night, and sonata (zaleplon) (1hr half-life) for nighttime awakenings. With sonata (zaleplon), its half-life is so short, you can take in middle of night and usually be okay in the morning.
Ambien (zolpidem) CR. You might want to try Ambien (zolpidem) cr that would do the same thing without having to split the tab or taking anything in theiddlenof the night.