we flew to Australia from Seattle when my son we 3yrs old. the flight from LAX left at 12.20am, as soon as we got on the flight I put him in his PJ's did his night time routine and he went to sleep.. aside from the location,,if your kiddo is a good sleeper while traveling I would go for it, if she needs the comfort and routine of a bed etc try a different flight... also if she is still in a car seat not a booster,, take the seat,, she is used to it and may sleep better that way ?