What can cause unconscious teeth clenching and grinding (bruxism) whilst asleep?

Clenching. Stress...The body subconsciously clenches and grinds in the sleep to try and alleviate the stress. A night guard is the best course of treatment.
Still unclear! While many dentists still believe that bruxism is related only to stress, the subject is controversial and still poorly understood. Other possible causes are abnormal alignment of upper and lower teeth, side effects from certain medications and most recently the big one: sleep problems!
Teeth clenching. Can be a habit, allergic reaction, medical ailment such as parkinson disease, trauma, stress. Some triggers include digestive ailments & anxiety, malalignment of teeth, ear ache or teething, or effect of some antidepressant medications.