Definitely take it beforehand. I'm actually at the other end of the spectrum - I have an incredible tolerance to all kinds of narcotics, sleeping pills and muscle relaxers. Things that most people can only take a half or small dose of and be knocked out, I can take three things as much and function normally. I used to take a cocktail of vicodin and flexeril (muscle relaxer) to work at a library when I had back problems and I never felt any mental affects at all.

That aside, I would still never try something new out on a plane, or probably at all. I totally got sleep paralyisis one time from taking Valium to help me sleep one night and I would not want to experience anything panic like on a plane.

PS - I'm also a female who is 5'3"!