There isn't a single answer to this question as it could be a list of things, such as stress, nightmares, or even mental disorders such as PTSD can be the cause of such.
Caffeine intake is also known to be a factor that can cause you to grind your teeth. Try to avoid caffeine before bed, and minimize intake during the day.
- The most common reason is your stress level. People tend to clench their teeth at night when a stressful situation bothers them. Without realizing it, you will just bite so hard that your muscles are hurt. Then when you are asleep, your jaws are moving so hard that the chewing surfaces are damaged.
Other reasons for teeth grinding while you are sleeping include:
- New medication or a change in medication
- Abnormal bite or crooked or missing teeth