What does it mean if i grind my teeth every night when i sleep?

Causing damage: The reason can be anything from stress to habit. More importantly, you're causing permanent damage when you do. Those are the only teeth you get forever. Wear a mouthguard and protect them until it doesn't happen anymore.

Additionally: In addition to Dr. Weibe's excellent answer, sometimes grinding can be caused by malocclusion. Another cause can be side effect of medication. Please see your Dentist for an evaluation and probably a protective night guard.

Causes???: Should see a dentist and possibly get to source of the grinding or have a custom mouthpiece constructed. If the teeth are worn down, this will change your bite, which in turn can cause muscle and TMJ problems.