I'm pretty stingy about handing out sleeping pills. They can be habit-forming, and if you get habituated, the baby will be addicted in utero and will have withdrawal problems after birth. There's also Danish data suggesting that when pregnant women use barbiturates (like sleeping pills) regularly they may lower their baby's IQ and cause problems with the child's bilirubin metabolism after birth (which can lead to jaundice or even brain damage).
That said, there's no problem with using an occasional sleep remedy if, say, you're flying over the ocean and want to get sleep; ask your doctor for specific advice. But if it's just an issue of being uncomfortable at night or routine insomnia, I don't recommend sleeping pills. Instead, empty your bladder, prop yourself up with pillows, and read or have a snack, then go back to sleep naturally.