It's called bruxism and unfortunately, it's genetic. My son does it, I do it, my parents BOTH did it while I was living with them. At 15 mos old, there's not a whole lot you can do unless you take him to the dentist and get a special mouth guard. There are over-the-counter ones, but they would be WAY too big for a toddler. Your dentist can specially fit a soft silicone mold to your son's teeth to prevent tooth damage. BUT - they may not be willing to do this until he has all 20 baby teeth because then you'd have to get a new one each time a new tooth comes in.
Oh, and my son does it during the day when he's thinking or frustrated, too. Count your blessings it's only when he's asleep!
at 6:49 PM on Jan. 17, 2009