Contrary to common belief, functional habits such as chewing are not the main cause of the wearing of teeth. Parafunctional habits are the most destructive forces for several reasons. Whereas teeth rarely come into contact during normal chewing, grinding of teeth may occur 1 - 4 hours in a 24 hour period, most often during sleep. The amount of force placed on teeth during functional habits is 20 - 80 psi (0.14 - 0.55 MPa), but the force can range from 300 - 3000 psi (2.07 - 20.7 MPa) during parafunctional habits. The direction of forces during functional habits are placed vertically along the long axis of teeth, which is the least harmful because of the anatomical structure of the attachment of teeth to the bone. On the other hand, parafunctional habits direct their forces horizontally. Normally, the temporomandibular joint (TMJ) acts as a class III lever, which helps to restrict the amount of force generated. Class I or class II levers may be created during bruxism, which generates more force from the same amount of muscle activity and subsequently delivers more force to the teeth.