Sleepless Pregnancy

For the past week or so sleep has been keeping itself at quite a distance.  Either I have a horrible time falling asleep (toss, turn, wriggle around, toss, turn, move the pillow, toss turn...you get the picture), fall asleep quickly and then wake up every 20-30 minutes, or sleep for a solid 4-5 hours and then wake up and do the toss, turn and repeat routine until I finally give up and get out of bed and go downstairs, trying to at least be productive if I can't sleep.  Tonight it's been one of those where I have a horrible time falling asleep (hence the reason I'm blogging at 1:30 in the morning).

Many of my friends who are mothers have informed me that this is my body's way of preparing me for the first few-several months of my baby's life.  They have also been kind enough to give me tips on how to get more sleep.  Some suggested napping (something I haven't done since I was 3, with the exception of the half-a-dozen times I took a power-nap during college after pulling an all-nighter), while others suggested taking Benadryl, Tylenol PM, or something of the like (that's safe to take during pregnancy).  I have a prescription for an antihistamine that I take at bedtime 2-3 times a week to keep my horrible almost-every-day headaches at bay, but even taking half of a pill seems to knock me out so hard that I'm groggy and almost worthless the following day.  I've considered trying to cut my pill into fourths, only taking one-fourth at a time in hopes that the grogginess won't be so severe. 
On the nights I do take my antihistamine I sleep quite soundly for a solid 8-10 hours.  I just need to figure out something to help me sleep the rest of the nights.  Any thoughts/suggestions on what I should do to try to get more sleep?