What Happened, Mr. Sandman?

For the past couple of nights, sleeping has not been what it used to be. Starting about 30 minutes after we put the kids down, and lasting for about 4 hours, either Blondie or Little Man wake up about every 30 minutes and start to cry. We usually go to them and replace a lost pacifier and this solves the problem, but why has it started? Possible teething pain? Changes in sleepwear due to warmer weather? We don't know why, but it would be nice if it would stop.