Bruxism is excessive teeth grinding or jaw clenching that is unrelated to normal functions such as eating or talking.

Estimates vary as to its prevalence - between 8% and 31% of the total population.

It's classified into two different types, 'awake' bruxism and 'sleeping' brxuism. It can lead to tooth wear, muscle aches, and headaches.

The causes are unknown, though stress, genetic factors and medication-use have been found to affect its frequency.

Bruxism is a parafunctional activity, but it is debated whether this represents a subconscious habit or is entirely involuntary. The relative importance of the various identified possible causative factors is also debated.

Source : Wikipedia