Why do my teeth snap together biting very hard down closed and sometimes biting my inner cheek uncontrollably in my sleep?

Malocclusion : Malocclusion between the upper and lower teeth is the most likely explanation. This can also be related to TMJ and bruxism which may explain the night time symptoms. Your dentist or oral surgeon can help.

Clenching and: Grinding (bruxism) are para functional habits that sadly many of us suffer from. Best to be evaluated by a dentist to see what your options are. Hope this helps.

Bruxism: 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.