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This issue is usually due to a nocturnal "bruxism" habit-- an unconscious clenching of the jaw and grinding of the teeth. The precise cause for bruxism is not known, but it is usually ascribed to increased muscle tone secondary to psychological or physiologic stress. It tends to manifest in episodes of a few days to a few weeks, alternating with periods of dormancy.
Bruxism is most commonly managed by the construction of a night guard appliance- a device constructed by your dentist, and worn over the teeth during sleep. This appliance keeps the upper and lower teeth from meeting; in so doing, it protects the teeth, temporomandibular joints
, and soft tissues from injury. Used continuously during sleep, a night guard appliance can sometimes abolish the bruxism habit. (Although less commonly used, stress management and muscle relaxation techniques, including bio-feedback and hypnosis, have been employed in the management of this habit.)
Be advised that cheek and tongue biting can also occur due to sharp or rough areas on the teeth, as may occur due to cavities
, lost fillings
, or tooth breakage. Poor tooth position can also cause biting of the soft tissues; this is also a factor when dentures
or other tooth prosthetic devices have improperly set teeth or retention clasps.
Regardless of the specifics, sudden onset of lip, cheek, or tongue biting should be assessed by your dentist, who will be able to identify the cause and provide remedial treatment.
Hope this helps...