The best time to probably ween off them is at night. Especially if you have something else to put you to sleep. If you do feel any odd effects it might be the lexapro kicking in. Keep that in mind too. But you sound like you are doing good. Just keep breaking the tablet doing smaller and smaller over time. Letting your body adjust to the lesser dose each time. That's if you don't just want to stop. It is probably a safer way.
Like mjbrown said, since you are on a very small dose you should not have a problem. Just avoid long term usage AT ALL COSTS!!!! I am saying this from experience that the withdrawals from long term usage are horrendous! I wish I had never started taking it, but I have panic disorder and when you are in the midst of a full blown panic attack nothing else seems to matter but getting out of the way it make you fell, like your dying. So i kept taking the xanax and years later i have to deal with trying to get out of this mess.
.5mg of xanax is a very small dose (if you are only taking it once a day), so you should have little if any problems with withdrawal effects - especially if you are tapering. if trouble sleeping still occurs when the lexapro kicks in, you might want to consider an exercise plan (if you don't routinely exercise already). a good 20-30 walk / run 3 times a week has practically cured my sleeping troubles.
All the best..