Adam Doesn't Sleep

Help!  I'm so tired.  In the last few weeks, Adam stopped sleeping.  In his twenty months of life, he has slept through the night a few times.  Most commonly he is up once or twice.  For the last week or so, he has averaged five times per night, ranging up to SEVEN times a few nights ago.  Tonight he has already been up four times, and it's 1:30.  He sits up and calls out at the top of his lungs, "Mama!  Mama!"  He keeps calling until you go in and tuck the covers around him again.  It's horrible.  It jolts you out of bed, and there is no restorative sleep.  Just napping until the next scream.  My head hurts.  I'm exhausted.  Why wouldn't you sleep yet at twenty months?  How many months until we sleep again?  After the fourth time per night, you start making bargains...I would double our mortgage if I could just sleep at night.