How come people grind their teeth when they are sleeping?

Many causes: Someone may grind their teeth for a number of reasons. Often, the problem occurs because the muscles of the face and jaw are trying to get the teeth in a comfortable position. The muscles may be in spasm, you may have a tmj/tmd problem, stress, infection. Best to see a dentist asap. Often thew cause is difficult to determine, so may have to treat the symptoms with a mouthpiece.

Sleep Disorder: Nighttime grinding or bruxism is consider a central nervous system sleep disorder. New studies are showing strong correlation of bruxism to obstructive sleep apnea. http://www.sciencedaily.com/releases/2009/11/091102171213.htm.